Comment #6 on issue 2677 by ronan.l...@gmail.com: subs(Eq) does not work http://code.google.com/p/sympy/issues/detail?id=2677
Equality is commutative, but the arguments of subs() aren't. You want to make expr.subs(Eq(x**2, y)) do something completely different from expr.subs(Eq(y, x**2)), contradicting the obvious fact that if x**2 equals y, then y equals x**2. And, by the way, y == x**2 doesn't imply x == sqrt(y), but that's probably tangential to this issue.
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