Comment #11 on issue 2022 by asmeurer: inconsistent behaviour of subs when
using non-commutative symbols
http://code.google.com/p/sympy/issues/detail?id=2022
The case of replacing exp(x) with y in exp(rat*x) is not a problem since
x is extractable from rat*x and int*x.
That's the beauty of "extract_multiplicatively":
I shouldn't have to use extract _multiplicatively to do it, because the exp
could be buried in an expression (I also have to think about how to do the
power extraction, which should be done automatically). I don't care if
subs uses extract_multiplicatevly internally, though. That should actually
make it smarter than it is now.
The problem with "try use all the algebraic hints it can" is that you are
getting away from "explicit is better than
implicit". The user should have to say whether they want u-substitution
(single symbol replaced with the
expression obtained by solving a key relationship between that symbol
and "u"), extractive substitution or
exact substitution.
Well, maybe it shouldn't be enabled by default. But I think there should
be some all=True keyword to subs that tries everything .
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