diff --git a/.appveyor.yml b/.appveyor.yml
index 27089cb..4f3132d 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -1,7 +1,6 @@
 version: "{build}"
 shallow_clone: true
 image:
-  - Visual Studio 2015
   - Visual Studio 2017
   - Visual Studio 2019 Preview
 platform:
diff --git a/.travis.yml b/.travis.yml
index 8f3cc8b..739cd3b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,14 +9,6 @@ matrix:
       dist: trusty
       addons: { apt: { sources: ubuntu-toolchain-r-test, packages: ["xorg-dev", "g++-8"] } }
       env: MATRIX_EVAL="CC=gcc-8 && CXX=g++-8"
-    - os: linux
-      dist: trusty
-      addons: { apt: { sources: ["ubuntu-toolchain-r-test", "llvm-toolchain-trusty-5.0"], packages: ["xorg-dev", "clang-5.0", "g++-7"] } }
-      env: MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0"
-    - os: linux
-      dist: trusty
-      addons: { apt: { sources: ["ubuntu-toolchain-r-test", "llvm-toolchain-trusty-6.0"], packages: ["xorg-dev", "clang-6.0", "g++-7"] } }
-      env: MATRIX_EVAL="CC=clang-6.0 && CXX=clang++-6.0"
     - os: linux
       dist: trusty
       addons: { apt: { sources: ["ubuntu-toolchain-r-test", "llvm-toolchain-trusty-7"], packages: ["xorg-dev", "clang-7", "g++-7"] } }
@@ -35,7 +27,6 @@ before_install:
   - if [ "$TRAVIS_OS_NAME" == 'osx' ]; then
     brew update;
     brew upgrade cmake;
-    brew install git-lfs;
     fi
   - if [ "$TRAVIS_OS_NAME" == 'linux' ]; then
     mkdir $HOME/cmake;
diff --git a/README.md b/README.md
index 7a1a839..eddf500 100644
--- a/README.md
+++ b/README.md
@@ -183,6 +183,7 @@ void clear();
 iterator erase(const_iterator iter);
 iterator erase(const_iterator first, const_iterator last);
 size_type erase(const key_type& key);
+template < typename K > size_type erase(const K& key);
 
 void swap(flat_set& other);
 ```
@@ -191,18 +192,31 @@ void swap(flat_set& other);
 
 ```cpp
 size_type count(const key_type& key) const;
+template < typename K > size_type count(const K& key) const;
 
 iterator find(const key_type& key);
 const_iterator find(const key_type& key) const;
 
+template < typename K > iterator find(const K& key);
+template < typename K > const_iterator find(const K& key) const;
+
 std::pair<iterator, iterator> equal_range(const key_type& key);
 std::pair<const_iterator, const_iterator> equal_range(const key_type& key) const;
 
+template < typename K > std::pair<iterator, iterator> equal_range(const K& key);
+template < typename K > std::pair<const_iterator, const_iterator> equal_range(const K& key) const;
+
 iterator lower_bound(const key_type& key);
 const_iterator lower_bound(const key_type& key) const;
 
+template < typename K > iterator lower_bound(const K& key);
+template < typename K > const_iterator lower_bound(const K& key) const;
+
 iterator upper_bound(const key_type& key);
 const_iterator upper_bound(const key_type& key) const;
+
+template < typename K > iterator upper_bound(const K& key);
+template < typename K > const_iterator upper_bound(const K& key) const;
 ```
 
 ### Observers
@@ -390,6 +404,9 @@ mapped_type& operator[](const key_type& key);
 
 mapped_type& at(const key_type& key);
 const mapped_type& at(const key_type& key) const;
+
+template < typename K > mapped_type& at(const K& key);
+template < typename K > const mapped_type& at(const K& key) const;
 ```
 
 ### Modifiers
@@ -414,6 +431,7 @@ void clear();
 iterator erase(const_iterator iter);
 iterator erase(const_iterator first, const_iterator last);
 size_type erase(const key_type& key);
+template < typename K > size_type erase(const K& key);
 
 void swap(flat_map& other)
 ```
@@ -422,18 +440,31 @@ void swap(flat_map& other)
 
 ```cpp
 size_type count(const key_type& key) const;
+template < typename K > size_type count(const K& key) const;
 
 iterator find(const key_type& key);
 const_iterator find(const key_type& key) const;
 
+template < typename K > iterator find(const K& key);
+template < typename K > const_iterator find(const K& key) const;
+
 std::pair<iterator, iterator> equal_range(const key_type& key);
 std::pair<const_iterator, const_iterator> equal_range(const key_type& key) const;
 
+template < typename K > std::pair<iterator, iterator> equal_range(const K& key);
+template < typename K > std::pair<const_iterator, const_iterator> equal_range(const K& key) const;
+
 iterator lower_bound(const key_type& key);
 const_iterator lower_bound(const key_type& key) const;
 
+template < typename K > iterator lower_bound(const K& key);
+template < typename K > const_iterator lower_bound(const K& key) const;
+
 iterator upper_bound(const key_type& key);
 const_iterator upper_bound(const key_type& key) const;
+
+template < typename K > iterator upper_bound(const K& key);
+template < typename K > const_iterator upper_bound(const K& key) const;
 ```
 
 ### Observers
@@ -635,6 +666,7 @@ void clear();
 iterator erase(const_iterator iter);
 iterator erase(const_iterator first, const_iterator last);
 size_type erase(const key_type& key);
+template < typename K > size_type erase(const K& key);
 
 void swap(flat_multiset& other);
 ```
@@ -643,18 +675,31 @@ void swap(flat_multiset& other);
 
 ```cpp
 size_type count(const key_type& key) const;
+template < typename K > size_type count(const K& key) const;
 
 iterator find(const key_type& key);
 const_iterator find(const key_type& key) const;
 
+template < typename K > iterator find(const K& key);
+template < typename K > const_iterator find(const K& key) const;
+
 std::pair<iterator, iterator> equal_range(const key_type& key);
 std::pair<const_iterator, const_iterator> equal_range(const key_type& key) const;
 
+template < typename K > std::pair<iterator, iterator> equal_range(const K& key);
+template < typename K > std::pair<const_iterator, const_iterator> equal_range(const K& key) const;
+
 iterator lower_bound(const key_type& key);
 const_iterator lower_bound(const key_type& key) const;
 
+template < typename K > iterator lower_bound(const K& key);
+template < typename K > const_iterator lower_bound(const K& key) const;
+
 iterator upper_bound(const key_type& key);
 const_iterator upper_bound(const key_type& key) const;
+
+template < typename K > iterator upper_bound(const K& key);
+template < typename K > const_iterator upper_bound(const K& key) const;
 ```
 
 ### Observers
@@ -842,6 +887,9 @@ mapped_type& operator[](const key_type& key);
 
 mapped_type& at(const key_type& key);
 const mapped_type& at(const key_type& key) const;
+
+template < typename K > mapped_type& at(const K& key);
+template < typename K > const mapped_type& at(const K& key) const;
 ```
 
 ### Modifiers
@@ -866,6 +914,7 @@ void clear();
 iterator erase(const_iterator iter);
 iterator erase(const_iterator first, const_iterator last);
 size_type erase(const key_type& key);
+template < typename K > size_type erase(const K& key);
 
 void swap(flat_multimap& other)
 ```
@@ -874,18 +923,31 @@ void swap(flat_multimap& other)
 
 ```cpp
 size_type count(const key_type& key) const;
+template < typename K > size_type count(const K& key) const;
 
 iterator find(const key_type& key);
 const_iterator find(const key_type& key) const;
 
+template < typename K > iterator find(const K& key);
+template < typename K > const_iterator find(const K& key) const;
+
 std::pair<iterator, iterator> equal_range(const key_type& key);
 std::pair<const_iterator, const_iterator> equal_range(const key_type& key) const;
 
+template < typename K > std::pair<iterator, iterator> equal_range(const K& key);
+template < typename K > std::pair<const_iterator, const_iterator> equal_range(const K& key) const;
+
 iterator lower_bound(const key_type& key);
 const_iterator lower_bound(const key_type& key) const;
 
+template < typename K > iterator lower_bound(const K& key);
+template < typename K > const_iterator lower_bound(const K& key) const;
+
 iterator upper_bound(const key_type& key);
 const_iterator upper_bound(const key_type& key) const;
+
+template < typename K > iterator upper_bound(const K& key);
+template < typename K > const_iterator upper_bound(const K& key) const;
 ```
 
 ### Observers
diff --git a/headers/flat.hpp/detail/is_transparent.hpp b/headers/flat.hpp/detail/is_transparent.hpp
new file mode 100644
index 0000000..2f6ec11
--- /dev/null
+++ b/headers/flat.hpp/detail/is_transparent.hpp
@@ -0,0 +1,23 @@
+/*******************************************************************************
+ * This file is part of the "https://github.com/blackmatov/flat.hpp"
+ * For conditions of distribution and use, see copyright notice in LICENSE.md
+ * Copyright (C) 2019, by Matvey Cherevko (blackmatov@gmail.com)
+ ******************************************************************************/
+
+#pragma once
+
+#include <type_traits>
+
+namespace flat_hpp::detail
+{
+    template < typename T, typename U, typename = void >
+    struct is_transparent
+    : std::false_type {};
+
+    template < typename T, typename U >
+    struct is_transparent<T, U, std::void_t<typename T::is_transparent>>
+    : std::true_type {};
+
+    template < typename T, typename U >
+    inline constexpr bool is_transparent_v = is_transparent<T, U>::value;
+}
diff --git a/headers/flat.hpp/detail/pair_compare.hpp b/headers/flat.hpp/detail/pair_compare.hpp
new file mode 100644
index 0000000..3d90c44
--- /dev/null
+++ b/headers/flat.hpp/detail/pair_compare.hpp
@@ -0,0 +1,62 @@
+/*******************************************************************************
+ * This file is part of the "https://github.com/blackmatov/flat.hpp"
+ * For conditions of distribution and use, see copyright notice in LICENSE.md
+ * Copyright (C) 2019, by Matvey Cherevko (blackmatov@gmail.com)
+ ******************************************************************************/
+
+#pragma once
+
+#include "is_transparent.hpp"
+
+namespace flat_hpp::detail
+{
+    template < typename Pair, typename Compare >
+    class pair_compare : public Compare {
+    public:
+        pair_compare() = default;
+
+        pair_compare(const Compare& compare)
+        : Compare(compare) {}
+
+        bool operator()(
+            const typename Pair::first_type& l,
+            const typename Pair::first_type& r) const
+        {
+            return Compare::operator()(l, r);
+        }
+
+        bool operator()(const Pair& l, const Pair& r) const {
+            return Compare::operator()(l.first, r.first);
+        }
+
+        bool operator()(
+            const typename Pair::first_type& l,
+            const Pair& r) const
+        {
+            return Compare::operator()(l, r.first);
+        }
+
+        bool operator()(
+            const Pair& l,
+            const typename Pair::first_type& r) const
+        {
+            return Compare::operator()(l.first, r);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            is_transparent_v<Compare, K>,
+            bool>
+        operator()(const K& l, const Pair& r) const {
+            return Compare::operator()(l, r.first);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            is_transparent_v<Compare, K>,
+            bool>
+        operator()(const Pair& l, const K& r) const {
+            return Compare::operator()(l.first, r);
+        }
+    };
+}
diff --git a/headers/flat.hpp/flat_map.hpp b/headers/flat.hpp/flat_map.hpp
index 54baa50..a6ddb38 100644
--- a/headers/flat.hpp/flat_map.hpp
+++ b/headers/flat.hpp/flat_map.hpp
@@ -15,55 +15,21 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/pair_compare.hpp"
+#include "detail/is_transparent.hpp"
+
 namespace flat_hpp
 {
-    namespace detail
-    {
-        template < typename Value, typename Compare >
-        class flat_map_compare : public Compare {
-        public:
-            flat_map_compare() = default;
-
-            flat_map_compare(const Compare& compare)
-            : Compare(compare) {}
-
-            bool operator()(
-                const typename Value::first_type& l,
-                const typename Value::first_type& r) const
-            {
-                return Compare::operator()(l, r);
-            }
-
-            bool operator()(
-                const typename Value::first_type& l,
-                const Value& r) const
-            {
-                return Compare::operator()(l, r.first);
-            }
-
-            bool operator()(
-                const Value& l,
-                const typename Value::first_type& r) const
-            {
-                return Compare::operator()(l.first, r);
-            }
-
-            bool operator()(const Value& l, const Value& r) const {
-                return Compare::operator()(l.first, r.first);
-            }
-        };
-    }
-
     template < typename Key
              , typename Value
              , typename Compare = std::less<Key>
              , typename Container = std::vector<std::pair<Key, Value>> >
     class flat_map
-        : private detail::flat_map_compare<
+        : private detail::pair_compare<
             typename Container::value_type,
             Compare>
     {
-        using base_type = detail::flat_map_compare<
+        using base_type = detail::pair_compare<
             typename Container::value_type,
             Compare>;
     public:
@@ -299,6 +265,30 @@ namespace flat_hpp
             throw std::out_of_range("flat_map::at: key not found");
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            mapped_type&>
+        at(const K& key) {
+            const iterator iter = find(key);
+            if ( iter != end() ) {
+                return iter->second;
+            }
+            throw std::out_of_range("flat_map::at: key not found");
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const mapped_type&>
+        at(const K& key) const {
+            const const_iterator iter = find(key);
+            if ( iter != end() ) {
+                return iter->second;
+            }
+            throw std::out_of_range("flat_map::at: key not found");
+        }
+
         std::pair<iterator, bool> insert(value_type&& value) {
             const iterator iter = lower_bound(value.first);
             return iter == end() || this->operator()(value, *iter)
@@ -362,7 +352,18 @@ namespace flat_hpp
         }
 
         size_type erase(const key_type& key) {
-            const iterator iter = find(key);
+            const const_iterator iter = find(key);
+            return iter != end()
+                ? (erase(iter), 1)
+                : 0;
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        erase(const K& key) {
+            const const_iterator iter = find(key);
             return iter != end()
                 ? (erase(iter), 1)
                 : 0;
@@ -384,16 +385,47 @@ namespace flat_hpp
             return iter != end() ? 1 : 0;
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        count(const K& key) const {
+            const const_iterator iter = find(key);
+            return iter != end() ? 1 : 0;
+        }
+
         iterator find(const key_type& key) {
             const iterator iter = lower_bound(key);
-            return iter != end() && !this->operator()(key, iter->first)
+            return iter != end() && !this->operator()(key, *iter)
                 ? iter
                 : end();
         }
 
         const_iterator find(const key_type& key) const {
             const const_iterator iter = lower_bound(key);
-            return iter != end() && !this->operator()(key, iter->first)
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        find(const K& key) {
+            const iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        find(const K& key) const {
+            const const_iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
                 ? iter
                 : end();
         }
@@ -408,6 +440,24 @@ namespace flat_hpp
             return std::equal_range(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<iterator, iterator>>
+        equal_range(const K& key) {
+            const base_type& comp = *this;
+            return std::equal_range(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<const_iterator, const_iterator>>
+        equal_range(const K& key) const {
+            const base_type& comp = *this;
+            return std::equal_range(begin(), end(), key, comp);
+        }
+
         iterator lower_bound(const key_type& key) {
             const base_type& comp = *this;
             return std::lower_bound(begin(), end(), key, comp);
@@ -418,6 +468,24 @@ namespace flat_hpp
             return std::lower_bound(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        lower_bound(const K& key) {
+            const base_type& comp = *this;
+            return std::lower_bound(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        lower_bound(const K& key) const {
+            const base_type& comp = *this;
+            return std::lower_bound(begin(), end(), key, comp);
+        }
+
         iterator upper_bound(const key_type& key) {
             const base_type& comp = *this;
             return std::upper_bound(begin(), end(), key, comp);
@@ -428,6 +496,24 @@ namespace flat_hpp
             return std::upper_bound(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        upper_bound(const K& key) {
+            const base_type& comp = *this;
+            return std::upper_bound(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        upper_bound(const K& key) const {
+            const base_type& comp = *this;
+            return std::upper_bound(begin(), end(), key, comp);
+        }
+
         key_compare key_comp() const {
             return *this;
         }
diff --git a/headers/flat.hpp/flat_multimap.hpp b/headers/flat.hpp/flat_multimap.hpp
index c20d395..aeccbbf 100644
--- a/headers/flat.hpp/flat_multimap.hpp
+++ b/headers/flat.hpp/flat_multimap.hpp
@@ -15,55 +15,21 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/pair_compare.hpp"
+#include "detail/is_transparent.hpp"
+
 namespace flat_hpp
 {
-    namespace detail
-    {
-        template < typename Value, typename Compare >
-        class flat_multimap_compare : public Compare {
-        public:
-            flat_multimap_compare() = default;
-
-            flat_multimap_compare(const Compare& compare)
-            : Compare(compare) {}
-
-            bool operator()(
-                const typename Value::first_type& l,
-                const typename Value::first_type& r) const
-            {
-                return Compare::operator()(l, r);
-            }
-
-            bool operator()(
-                const typename Value::first_type& l,
-                const Value& r) const
-            {
-                return Compare::operator()(l, r.first);
-            }
-
-            bool operator()(
-                const Value& l,
-                const typename Value::first_type& r) const
-            {
-                return Compare::operator()(l.first, r);
-            }
-
-            bool operator()(const Value& l, const Value& r) const {
-                return Compare::operator()(l.first, r.first);
-            }
-        };
-    }
-
     template < typename Key
              , typename Value
              , typename Compare = std::less<Key>
              , typename Container = std::vector<std::pair<Key, Value>> >
     class flat_multimap
-        : private detail::flat_multimap_compare<
+        : private detail::pair_compare<
             typename Container::value_type,
             Compare>
     {
-        using base_type = detail::flat_multimap_compare<
+        using base_type = detail::pair_compare<
             typename Container::value_type,
             Compare>;
     public:
@@ -299,6 +265,30 @@ namespace flat_hpp
             throw std::out_of_range("flat_multimap::at: key not found");
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            mapped_type&>
+        at(const K& key) {
+            const iterator iter = find(key);
+            if ( iter != end() ) {
+                return iter->second;
+            }
+            throw std::out_of_range("flat_multimap::at: key not found");
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const mapped_type&>
+        at(const K& key) const {
+            const const_iterator iter = find(key);
+            if ( iter != end() ) {
+                return iter->second;
+            }
+            throw std::out_of_range("flat_multimap::at: key not found");
+        }
+
         iterator insert(value_type&& value) {
             const iterator iter = upper_bound(value.first);
             return data_.insert(iter, std::move(value));
@@ -364,6 +354,17 @@ namespace flat_hpp
             return r;
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        erase(const K& key) {
+            const auto p = equal_range(key);
+            size_type r = std::distance(p.first, p.second);
+            erase(p.first, p.second);
+            return r;
+        }
+
         void swap(flat_multimap& other)
             noexcept(std::is_nothrow_swappable_v<base_type>
                 && std::is_nothrow_swappable_v<container_type>)
@@ -380,16 +381,47 @@ namespace flat_hpp
             return std::distance(p.first, p.second);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        count(const K& key) const {
+            const auto p = equal_range(key);
+            return std::distance(p.first, p.second);
+        }
+
         iterator find(const key_type& key) {
             const iterator iter = lower_bound(key);
-            return iter != end() && !this->operator()(key, iter->first)
+            return iter != end() && !this->operator()(key, *iter)
                 ? iter
                 : end();
         }
 
         const_iterator find(const key_type& key) const {
             const const_iterator iter = lower_bound(key);
-            return iter != end() && !this->operator()(key, iter->first)
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        find(const K& key) {
+            const iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        find(const K& key) const {
+            const const_iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
                 ? iter
                 : end();
         }
@@ -404,6 +436,24 @@ namespace flat_hpp
             return std::equal_range(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<iterator, iterator>>
+        equal_range(const K& key) {
+            const base_type& comp = *this;
+            return std::equal_range(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<const_iterator, const_iterator>>
+        equal_range(const K& key) const {
+            const base_type& comp = *this;
+            return std::equal_range(begin(), end(), key, comp);
+        }
+
         iterator lower_bound(const key_type& key) {
             const base_type& comp = *this;
             return std::lower_bound(begin(), end(), key, comp);
@@ -414,6 +464,24 @@ namespace flat_hpp
             return std::lower_bound(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        lower_bound(const K& key) {
+            const base_type& comp = *this;
+            return std::lower_bound(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        lower_bound(const K& key) const {
+            const base_type& comp = *this;
+            return std::lower_bound(begin(), end(), key, comp);
+        }
+
         iterator upper_bound(const key_type& key) {
             const base_type& comp = *this;
             return std::upper_bound(begin(), end(), key, comp);
@@ -424,6 +492,24 @@ namespace flat_hpp
             return std::upper_bound(begin(), end(), key, comp);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        upper_bound(const K& key) {
+            const base_type& comp = *this;
+            return std::upper_bound(begin(), end(), key, comp);
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        upper_bound(const K& key) const {
+            const base_type& comp = *this;
+            return std::upper_bound(begin(), end(), key, comp);
+        }
+
         key_compare key_comp() const {
             return *this;
         }
diff --git a/headers/flat.hpp/flat_multiset.hpp b/headers/flat.hpp/flat_multiset.hpp
index e6e9111..6fdd395 100644
--- a/headers/flat.hpp/flat_multiset.hpp
+++ b/headers/flat.hpp/flat_multiset.hpp
@@ -15,6 +15,8 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/is_transparent.hpp"
+
 namespace flat_hpp
 {
     template < typename Key
@@ -279,6 +281,17 @@ namespace flat_hpp
             return r;
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        erase(const K& key) {
+            const auto p = equal_range(key);
+            size_type r = std::distance(p.first, p.second);
+            erase(p.first, p.second);
+            return r;
+        }
+
         void swap(flat_multiset& other)
             noexcept(std::is_nothrow_swappable_v<base_type>
                 && std::is_nothrow_swappable_v<container_type>)
@@ -295,6 +308,15 @@ namespace flat_hpp
             return std::distance(p.first, p.second);
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        count(const K& key) const {
+            const auto p = equal_range(key);
+            return std::distance(p.first, p.second);
+        }
+
         iterator find(const key_type& key) {
             const iterator iter = lower_bound(key);
             return iter != end() && !this->operator()(key, *iter)
@@ -309,6 +331,28 @@ namespace flat_hpp
                 : end();
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        find(const K& key) {
+            const iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        find(const K& key) const {
+            const const_iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
         std::pair<iterator, iterator> equal_range(const key_type& key) {
             return std::equal_range(begin(), end(), key, key_comp());
         }
@@ -317,6 +361,22 @@ namespace flat_hpp
             return std::equal_range(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<iterator, iterator>>
+        equal_range(const K& key) {
+            return std::equal_range(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<const_iterator, const_iterator>>
+        equal_range(const K& key) const {
+            return std::equal_range(begin(), end(), key, key_comp());
+        }
+
         iterator lower_bound(const key_type& key) {
             return std::lower_bound(begin(), end(), key, key_comp());
         }
@@ -325,6 +385,22 @@ namespace flat_hpp
             return std::lower_bound(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        lower_bound(const K& key) {
+            return std::lower_bound(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        lower_bound(const K& key) const {
+            return std::lower_bound(begin(), end(), key, key_comp());
+        }
+
         iterator upper_bound(const key_type& key) {
             return std::upper_bound(begin(), end(), key, key_comp());
         }
@@ -333,6 +409,22 @@ namespace flat_hpp
             return std::upper_bound(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        upper_bound(const K& key) {
+            return std::upper_bound(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        upper_bound(const K& key) const {
+            return std::upper_bound(begin(), end(), key, key_comp());
+        }
+
         key_compare key_comp() const {
             return *this;
         }
diff --git a/headers/flat.hpp/flat_set.hpp b/headers/flat.hpp/flat_set.hpp
index d5cf454..21e06ab 100644
--- a/headers/flat.hpp/flat_set.hpp
+++ b/headers/flat.hpp/flat_set.hpp
@@ -15,6 +15,8 @@
 #include <type_traits>
 #include <initializer_list>
 
+#include "detail/is_transparent.hpp"
+
 namespace flat_hpp
 {
     template < typename Key
@@ -277,7 +279,18 @@ namespace flat_hpp
         }
 
         size_type erase(const key_type& key) {
-            const iterator iter = find(key);
+            const const_iterator iter = find(key);
+            return iter != end()
+                ? (erase(iter), 1)
+                : 0;
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        erase(const K& key) {
+            const const_iterator iter = find(key);
             return iter != end()
                 ? (erase(iter), 1)
                 : 0;
@@ -299,6 +312,15 @@ namespace flat_hpp
             return iter != end() ? 1 : 0;
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            size_type>
+        count(const K& key) const {
+            const const_iterator iter = find(key);
+            return iter != end() ? 1 : 0;
+        }
+
         iterator find(const key_type& key) {
             const iterator iter = lower_bound(key);
             return iter != end() && !this->operator()(key, *iter)
@@ -313,6 +335,28 @@ namespace flat_hpp
                 : end();
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        find(const K& key) {
+            const iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        find(const K& key) const {
+            const const_iterator iter = lower_bound(key);
+            return iter != end() && !this->operator()(key, *iter)
+                ? iter
+                : end();
+        }
+
         std::pair<iterator, iterator> equal_range(const key_type& key) {
             return std::equal_range(begin(), end(), key, key_comp());
         }
@@ -321,6 +365,22 @@ namespace flat_hpp
             return std::equal_range(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<iterator, iterator>>
+        equal_range(const K& key) {
+            return std::equal_range(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            std::pair<const_iterator, const_iterator>>
+        equal_range(const K& key) const {
+            return std::equal_range(begin(), end(), key, key_comp());
+        }
+
         iterator lower_bound(const key_type& key) {
             return std::lower_bound(begin(), end(), key, key_comp());
         }
@@ -329,6 +389,22 @@ namespace flat_hpp
             return std::lower_bound(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        lower_bound(const K& key) {
+            return std::lower_bound(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        lower_bound(const K& key) const {
+            return std::lower_bound(begin(), end(), key, key_comp());
+        }
+
         iterator upper_bound(const key_type& key) {
             return std::upper_bound(begin(), end(), key, key_comp());
         }
@@ -337,6 +413,22 @@ namespace flat_hpp
             return std::upper_bound(begin(), end(), key, key_comp());
         }
 
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            iterator>
+        upper_bound(const K& key) {
+            return std::upper_bound(begin(), end(), key, key_comp());
+        }
+
+        template < typename K >
+        std::enable_if_t<
+            detail::is_transparent_v<Compare, K>,
+            const_iterator>
+        upper_bound(const K& key) const {
+            return std::upper_bound(begin(), end(), key, key_comp());
+        }
+
         key_compare key_comp() const {
             return *this;
         }
diff --git a/untests/flat_map_tests.cpp b/untests/flat_map_tests.cpp
index ced86ce..45fbd38 100644
--- a/untests/flat_map_tests.cpp
+++ b/untests/flat_map_tests.cpp
@@ -9,6 +9,9 @@
 
 #include <deque>
 
+#include <string>
+#include <string_view>
+
 #include <flat.hpp/flat_map.hpp>
 using namespace flat_hpp;
 
@@ -54,6 +57,10 @@ namespace
 }
 
 TEST_CASE("flat_map") {
+    SECTION("detail") {
+        STATIC_REQUIRE(detail::is_transparent<std::less<>, int>::value);
+        STATIC_REQUIRE_FALSE(detail::is_transparent<std::less<int>, int>::value);
+    }
     SECTION("sizeof") {
         REQUIRE(sizeof(flat_map<int, unsigned>) == sizeof(std::vector<std::pair<int, unsigned>>));
 
@@ -404,6 +411,23 @@ TEST_CASE("flat_map") {
             REQUIRE(my_as_const(s0).lower_bound(7) == s0.cbegin() + 3);
         }
     }
+    SECTION("heterogeneous") {
+        flat_map<std::string, int, std::less<>> s0{{"hello", 42}, {"world", 84}};
+        REQUIRE(s0.find(std::string_view("hello")) == s0.begin());
+        REQUIRE(my_as_const(s0).find(std::string_view("world")) == s0.begin() + 1);
+        REQUIRE(s0.find(std::string_view("42")) == s0.end());
+        REQUIRE(my_as_const(s0).find(std::string_view("42")) == s0.cend());
+
+        REQUIRE(my_as_const(s0).count(std::string_view("hello")) == 1);
+        REQUIRE(my_as_const(s0).count(std::string_view("hello_42")) == 0);
+
+        REQUIRE(s0.upper_bound(std::string_view("hello")) == s0.begin() + 1);
+        REQUIRE(my_as_const(s0).upper_bound(std::string_view("hello")) == s0.begin() + 1);
+
+        REQUIRE(s0.erase(std::string_view("hello")) == 1);
+        REQUIRE(s0.at(std::string_view("world")) == 84);
+        REQUIRE(my_as_const(s0).at(std::string_view("world")) == 84);
+    }
     SECTION("observers") {
         struct my_less {
             int i;
diff --git a/untests/flat_multimap_tests.cpp b/untests/flat_multimap_tests.cpp
index ccaf5bd..0c9953a 100644
--- a/untests/flat_multimap_tests.cpp
+++ b/untests/flat_multimap_tests.cpp
@@ -9,6 +9,9 @@
 
 #include <deque>
 
+#include <string>
+#include <string_view>
+
 #include <flat.hpp/flat_multimap.hpp>
 using namespace flat_hpp;
 
@@ -54,6 +57,10 @@ namespace
 }
 
 TEST_CASE("flat_multimap") {
+    SECTION("detail") {
+        STATIC_REQUIRE(detail::is_transparent<std::less<>, int>::value);
+        STATIC_REQUIRE_FALSE(detail::is_transparent<std::less<int>, int>::value);
+    }
     SECTION("sizeof") {
         REQUIRE(sizeof(flat_multimap<int, unsigned>) == sizeof(std::vector<std::pair<int, unsigned>>));
 
@@ -406,6 +413,23 @@ TEST_CASE("flat_multimap") {
             REQUIRE(my_as_const(s0).lower_bound(7) == s0.cbegin() + 4);
         }
     }
+    SECTION("heterogeneous") {
+        flat_multimap<std::string, int, std::less<>> s0{{"hello", 42}, {"world", 84}};
+        REQUIRE(s0.find(std::string_view("hello")) == s0.begin());
+        REQUIRE(my_as_const(s0).find(std::string_view("world")) == s0.begin() + 1);
+        REQUIRE(s0.find(std::string_view("42")) == s0.end());
+        REQUIRE(my_as_const(s0).find(std::string_view("42")) == s0.cend());
+
+        REQUIRE(my_as_const(s0).count(std::string_view("hello")) == 1);
+        REQUIRE(my_as_const(s0).count(std::string_view("hello_42")) == 0);
+
+        REQUIRE(s0.upper_bound(std::string_view("hello")) == s0.begin() + 1);
+        REQUIRE(my_as_const(s0).upper_bound(std::string_view("hello")) == s0.begin() + 1);
+
+        REQUIRE(s0.erase(std::string_view("hello")) == 1);
+        REQUIRE(s0.at(std::string_view("world")) == 84);
+        REQUIRE(my_as_const(s0).at(std::string_view("world")) == 84);
+    }
     SECTION("observers") {
         struct my_less {
             int i;
diff --git a/untests/flat_multiset_tests.cpp b/untests/flat_multiset_tests.cpp
index 669d799..9e7833f 100644
--- a/untests/flat_multiset_tests.cpp
+++ b/untests/flat_multiset_tests.cpp
@@ -9,6 +9,9 @@
 
 #include <deque>
 
+#include <string>
+#include <string_view>
+
 #include <flat.hpp/flat_multiset.hpp>
 using namespace flat_hpp;
 
@@ -54,6 +57,10 @@ namespace
 }
 
 TEST_CASE("flat_multiset") {
+    SECTION("detail") {
+        STATIC_REQUIRE(detail::is_transparent<std::less<>, int>::value);
+        STATIC_REQUIRE_FALSE(detail::is_transparent<std::less<int>, int>::value);
+    }
     SECTION("sizeof") {
         REQUIRE(sizeof(flat_multiset<int>) == sizeof(std::vector<int>));
 
@@ -382,6 +389,21 @@ TEST_CASE("flat_multiset") {
             REQUIRE(my_as_const(s0).lower_bound(7) == s0.cbegin() + 4);
         }
     }
+    SECTION("heterogeneous") {
+        flat_multiset<std::string, std::less<>> s0{"hello", "world"};
+        REQUIRE(s0.find(std::string_view("hello")) == s0.begin());
+        REQUIRE(my_as_const(s0).find(std::string_view("world")) == s0.begin() + 1);
+        REQUIRE(s0.find(std::string_view("42")) == s0.end());
+        REQUIRE(my_as_const(s0).find(std::string_view("42")) == s0.cend());
+
+        REQUIRE(my_as_const(s0).count(std::string_view("hello")) == 1);
+        REQUIRE(my_as_const(s0).count(std::string_view("hello_42")) == 0);
+
+        REQUIRE(s0.upper_bound(std::string_view("hello")) == s0.begin() + 1);
+        REQUIRE(my_as_const(s0).upper_bound(std::string_view("hello")) == s0.begin() + 1);
+
+        REQUIRE(s0.erase(std::string_view("hello")) == 1);
+    }
     SECTION("observers") {
         struct my_less {
             int i;
diff --git a/untests/flat_set_tests.cpp b/untests/flat_set_tests.cpp
index 6029598..83dbd23 100644
--- a/untests/flat_set_tests.cpp
+++ b/untests/flat_set_tests.cpp
@@ -9,6 +9,9 @@
 
 #include <deque>
 
+#include <string>
+#include <string_view>
+
 #include <flat.hpp/flat_set.hpp>
 using namespace flat_hpp;
 
@@ -54,6 +57,10 @@ namespace
 }
 
 TEST_CASE("flat_set") {
+    SECTION("detail") {
+        STATIC_REQUIRE(detail::is_transparent<std::less<>, int>::value);
+        STATIC_REQUIRE_FALSE(detail::is_transparent<std::less<int>, int>::value);
+    }
     SECTION("sizeof") {
         REQUIRE(sizeof(flat_set<int>) == sizeof(std::vector<int>));
 
@@ -380,6 +387,21 @@ TEST_CASE("flat_set") {
             REQUIRE(my_as_const(s0).lower_bound(7) == s0.cbegin() + 3);
         }
     }
+    SECTION("heterogeneous") {
+        flat_set<std::string, std::less<>> s0{"hello", "world"};
+        REQUIRE(s0.find(std::string_view("hello")) == s0.begin());
+        REQUIRE(my_as_const(s0).find(std::string_view("world")) == s0.begin() + 1);
+        REQUIRE(s0.find(std::string_view("42")) == s0.end());
+        REQUIRE(my_as_const(s0).find(std::string_view("42")) == s0.cend());
+
+        REQUIRE(my_as_const(s0).count(std::string_view("hello")) == 1);
+        REQUIRE(my_as_const(s0).count(std::string_view("hello_42")) == 0);
+
+        REQUIRE(s0.upper_bound(std::string_view("hello")) == s0.begin() + 1);
+        REQUIRE(my_as_const(s0).upper_bound(std::string_view("hello")) == s0.begin() + 1);
+
+        REQUIRE(s0.erase(std::string_view("hello")) == 1);
+    }
     SECTION("observers") {
         struct my_less {
             int i;