I don't think it's really about "simple" functions per se. Bidirectionality is useful where function inversion is meaningful (i.e. not for hashing) and where t

I don't think it's really about "simple" functions per se. Bidirectionality is useful where function inversion is meaningful (i.e. not for hashing) and where the domain of the function is explicitly finite or otherwise bounded by constraints. That's exactly the situation when querying databases. Such queries can get quite complicated. Agreed that it's not that useful for GP programming, but it's very useful for databases and constraint management systems.