Comment #5 on issue 2476 by asmeurer: nth order Derivative http://code.google.com/p/sympy/issues/detail?id=2476
This would be inconsistent with f(x).diff(x).subs(x, y) == f(y).diff(y).
I didn't know we allowed that. Now I'm not really sure if your solution or my solution is very good.
One thing that I can say right now is that I'm pretty sure that f(x).diff(x, a).subs(a, 3) should NOT give f(x).diff(x, 3), because the two are similar only by coincidence of syntax.
I suppose it should just perform whatever the correct mathematical substitution is, taking the chain rule into consideration. Is Subs(Derivative(x**3, x, a), a, 3) that? It's hard for me to tell because the original expression is just 0 to begin with.
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