def _del_ins_lengths(self, ilen):
if self.ref is not None and self.ref != "<STR_LIT>":<EOL><INDENT>assert len(self.ref) == ilen<EOL><DEDENT>return (<NUM_LIT:0>, ilen)<EOL>
returns (del_len, ins_len).
        Unspecified ref or alt returns None for del_len or ins_len respectively.