code: def one_of(self, chset: str) -> str:
          
          res = self.peek()
          if res in chset:
              self.offset += 1
              return res
          raise UnexpectedInput(self, "one of " + chset)
docs: Parse one character form the specified set.
  
          Args:
              chset: string of characters to try as alternatives.
  
          Returns:
              The character that was actually matched.
  
          Raises:
              UnexpectedInput: If the next character is not in `chset`.