Richard,

Have you considered splitting the interface into two separate classes,
each with the same protocol?  That is, rather than having (public) #source
and #target versions of every method, have source and target wrapper
objects (perspectives?) that are backed by the same data.  I would guess
that many users of the BinaryRelation are always going to be navigating in
one direction or the other - if they were handed the appropiate
perspective, they wouldn't even necessarily have to realize they were
using anything but a dictionary.

I suppose this makes more sense as well as rather than instead of the
current interface, but it would IMO add a lot to the usability.

Avi