Comment #10 on issue 1525 by asmeurer: No integration by substitution
http://code.google.com/p/sympy/issues/detail?id=1525
But that's just plain wrong. The variable of differentiation is not like
the variable of integration. You can't just change it. f(x).diff(x) is
different from f(y).diff(y), i.e.,
In [59]: f(x).diff(x).free_symbols
Out[59]: set(x)
This also incorrect because you technically have the derivative evaluated
at the point 2*x, not with respect to 2*x, but that problem also exists
with the Derivative(f(g(x)), g(x)) thing, and would require fixing issue
1620 to clear up completely.
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