Comment #6 on issue 2049 by asmeurer: Derivative(f(x), x).subs(f(x), y)
gives 0 instead of Derivative(y, x)
http://code.google.com/p/sympy/issues/detail?id=2049
It will be the derivatives of f(x), up to the order of the ODE, so
expr.subs(f(x).diff(x, order), newterm.diff(x, order))
expr.subs(f(x).diff(x, order - 1), newterm.diff(x, order - 1))
…
expr.subs(f(x).diff(x, 2), newterm.diff(x, 2))
expr.subs(f(x).diff(x, 1), newterm.diff(x, 1))
expr.subs(f(x), newterm)
I forget what the newterm's are exactly, but it's whatever is currently
being passed to subs.
By the way, doit() shouldn't be so slow. We should look into that as well.
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