Comment #47 on issue 1816 by ronan.l...@gmail.com: Adding partial derivatives and taking derivatives with respect to functions
http://code.google.com/p/sympy/issues/detail?id=1816
Yes, I know that the implementation uses ._diff_wrt, but the resulting behaviour appears completely arbitrary from the point of view of an end-user. What's special about instances of Function that makes them the only expressions allowed in .diff()? Or in concrete terms, why is it that diff(expr, cos(2*x)) works but diff(expr, 2*cos(x)) raises?
BTW, if I understand correctly, diff(diff(f(x), x), f(x)) gives 0 not because f'(x) is considered a separate symbol but because f(x) is considered as a symbol independent of x.
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