YAML file: a.yml
  YAML document
    PsiElement(---)('---')
    PsiElement(Eol)('\n')
    YAML mapping
      YAML key value
        PsiElement(scalar key)('foo')
        PsiElement(:)(':')
        PsiWhiteSpace(' ')
        YAML hash
          PsiElement({)('{')
          PsiElement(})('}')
      PsiElement(Eol)('\n')
      YAML key value
        PsiElement(scalar key)('bar')
        PsiElement(:)(':')
        PsiWhiteSpace(' ')
        YAML quoted text
          PsiElement(scalar dstring)('"baz"')