return encoded representation

def binary(self):
        """
        return encoded representation
        """
        length = len(self.id) / 4
        if length == 0:
            return (
                b_chr(_TAG_REFERENCE_EXT) +
                self.node.binary() + self.id + self.creation
            )
        elif length <= 65535:
            creation_size = len(self.creation)
            if creation_size == 1:
                return (
                    b_chr(_TAG_NEW_REFERENCE_EXT) +
                    struct.pack(b'>H', length) +
                    self.node.binary() + self.creation + self.id
                )
            elif creation_size == 4:
                return (
                    b_chr(_TAG_NEWER_REFERENCE_EXT) +
                    struct.pack(b'>H', length) +
                    self.node.binary() + self.creation + self.id
                )
            else:
                raise OutputException('unknown reference type')
        else:
            raise OutputException('uint16 overflow')