Comment #20 on issue 1525 by [email protected]: No integration by
substitution
http://code.google.com/p/sympy/issues/detail?id=1525
I wonder if f(g(x)).diff(g(x)) could be stored as D(f(u), u, {u:g(x)}) and
the subs method of Derivative would look for a substitution {u;g(x)} that
could be done if the f is changed so that the derivative would then be
known, e.g.
D(f(u), u, {u;g(x)}).subs(f, sin) -> cos(u).subs({u:g(x)}) -> cos(g(x))
-- done automatically by subs ->
So it would mean just storing one extra item in the args, the replacement
dictionary.
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