Reading the patch, I still don't see how to invoke general derivative
test code for a unary function (and it doesn't appear to happen
automatically either). So I don't understand your comment.
The existing checks are for analytic derivatives. If you also want to
check numerical derivatives, that would certainly make sense. But to do
this properly requires something like dfridr from Numerical Recipes. I
have not done this so far because the NR license prohibits us from
including this code directly in IMP.