code: def letter():
      '''Parse a letter in alphabet.'''
      @Parser
      def letter_parser(text, index=0):
          if index < len(text) and text[index].isalpha():
              return Value.success(index + 1, text[index])
          else:
              return Value.failure(index, 'a letter')
      return letter_parsef letter():
      '''Parse a letter in alphabet.'''
      @Parser
      def letter_parser(text, index=0):
          if index < len(text) and text[index].isalpha():
              return Value.success(index + 1, text[index])
          else:
              return Value.failure(index, 'a letter')
      return letter_parser
docs: Parse a letter in alphabet.