YAML file: a.yml
  YAML document
    YAML mapping
      PsiElement(indent)('    ')
      YAML key value
        PsiElement(scalar key)('key')
        PsiElement(:)(':')
        PsiWhiteSpace(' ')
        YAML plain scalar text
          PsiElement(text)('ttt')
      PsiElement(Eol)('\n')
      PsiElement(indent)('    ')
      YAML key value
        PsiElement(scalar key)('ahahah')
        PsiElement(:)(':')
        PsiWhiteSpace(' ')
        YAML plain scalar text
          PsiElement(text)('ppp')