I added some python code to check that the derivative matches (more or
less) the slope of the values on a unary function. Currently this is
just applied to harmonic, which is not extremely useful (but I do use it
on other not-yet submitted unary functions).
The patch seems to touch a lot of stuff that's entirely unrelated to
that. But I don't know why you want to check the derivatives anyway - I
already added code to check derivatives for *all* the unary functions in
r473.