Updates:
Blockedon: 1688
Comment #1 on issue 1604 by asmeurer: Misapplication of integration by parts
http://code.google.com/p/sympy/issues/detail?id=1604
This ends up being the source of the error:
In [2]: diff(g(x), x).subs(g(x), y)
Out[2]: 0
It isn't incorrect, but the Risch algorithm doesn't expect a function to
behave like that. So obviously, the
integration engine should just quit early if it finds arbitrary function
like g(x) that depends on the integration
variable. If you want to manipulate things like that, you need to use
dsolve().
Unfortunately, this is not easy to do, because there is no definite way to
differentiate between f(x) and exp(x) as
functions. See issue 1688. I suppose for now, the work around can be to
have heurish bail when it sees a
Derivative.
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