If I recall correctly, constructing the inverse was time consuming, and the
way I had it coded before aimed to balance the memory and computational
loads. Since the "standard" inverse is used in many tests (I think?), it
would probably increase the computation time more than is worth the savings
in memory. At least that's what I seem to remember -- feel free to conduct
your own timing tests. If you do "easy_install nose-timer" and then run
"nosetests --with-timer" you can check easily.