YAML file: BlockMapping.yml
  YAML document
    YAML mapping
      YAML key value
        PsiElement(scalar key)('foo')
        PsiElement(:)(':')
        PsiElement(Eol)('\n')
        PsiElement(indent)('  ')
        YAML mapping
          YAML key value
            PsiElement(scalar key)('bar')
            PsiElement(:)(':')
            PsiWhiteSpace(' ')
            YAML plain scalar text
              PsiElement(text)('baz')