>>> diff sin 0 1.0
>>> cos 0 1.0
diff' sin == sin &&& cos diff' f = f &&& d f
>>> diff' sin 0 (0.0,1.0)
>>> diff' exp 0 (1.0,1.0)
>>> diffF (\a -> [sin a, cos a]) 0 [1.0,-0.0]
>>> diffF' (\a -> [sin a, cos a]) 0 [(0.0,1.0),(1.0,-0.0)]