It would be nice to have a feature for specifying two strings, one in from-file and one in to-file, which should be considered to match. Thus, if the two strings are `foo' and `bar', then if two lines differ only in that `foo' in file 1 corresponds to `bar' in file 2, the lines are treated as identical.
It is not clear how general this feature can or should be, or what syntax should be used for it.