How about: "Subclasses should override the describing_function, __call__, and _isstatic methods."

It looks like __call__ 's inputs and outputs must be scalar, since __call__ is allowed to change instance memory, so the sequence of calls matters.

Can describing_function be called with a numpy.ndarray of amplitude values? This seems like a reasonable thing to want to do.

If I understand correctly, if __call__ updates any instance state, then _isstatic must return False -- it would be good to mention that too.