Comment #8 on issue 2022 by smichr: inconsistent behaviour of subs when using non-commutative symbols
http://code.google.com/p/sympy/issues/detail?id=2022
But in that case you really aren't eliminating anything as you have x*y as the result (though it appears as y**(3/2). This is where the overlapping (present) behavior of subs is causing problems. The user should have done a subs(x, sqrt(y)) if that is what they wanted.
This is where I would lobby for a subs syntax to handle an explicit u-substitution, something like eq.subs([x], Eq(x**2, y)) to mean "replace x with what you get when you solve x**2 = y". (There are two solutions, perhaps two results should be returned and the user can pick which one they want.)
Has anyone suggested a filter option to subs where you can tell it what sort of expression to make the substitution in? so, for example
(x + x**3).subs([x], Eq(x**2, y), filter = lambda w: w.is_Pow) => x + y**(3/2)
But except for the proof of concept that such a replacement is possible, would anyone ever want to do such a thing?
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