Comment #5 on issue 2049 by smichr: 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
The selectively-doit approach that I take changes the run time from about 800 to 1400, the time mainly increasing in ode. This needs a better handling of the derivatives.
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