code: def one_of(s):
      '''Parser a char from specified string.'''
      @Parser
      def one_of_parser(text, index=0):
          if index < len(text) and text[index] in s:
              return Value.success(index + 1, text[index])
          else:
              return Value.failure(index, 'one of {}'.format(s))
      return one_of_parsef one_of(s):
      '''Parser a char from specified string.'''
      @Parser
      def one_of_parser(text, index=0):
          if index < len(text) and text[index] in s:
              return Value.success(index + 1, text[index])
          else:
              return Value.failure(index, 'one of {}'.format(s))
      return one_of_parser
docs: Parser a char from specified string.