Comment #1 on issue 2026 by asmeurer: Exact substitution http://code.google.com/p/sympy/issues/detail?id=2026
By the way, a while back Chris and I were talking about this on IRC, and we decided that expr.subs(old, new, exact=True) should work if and only if old is in expr.atoms(old.__class__). In other words, this excludes things like (x*y*z).subs(x*y, a, exact=True) and (x + y + z).subs(x + y, a) from working. The reasoning is that if those things worked, it would be inconsistent with (x**2*z).subs(x*z, a) not working, which of course it must not, or else we get again the subs-power manipulation that we are trying to avoid here.
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