code: def keyword(self) -> Tuple[Optional[str], str]:
          
          i1 = self.yang_identifier()
          if self.peek() == ":":
              self.offset += 1
              i2 = self.yang_identifier()
              return (i1, i2)
          return (None, i1)
docs: Parse a YANG statement keyword.
  
          Raises:
              EndOfInput: If past the end of input.
              UnexpectedInput: If no syntactically correct keyword is found.