Comment #19 on issue 2026 by asmeurer: Exact, algebraic, and integer_power substitution
http://code.google.com/p/sympy/issues/detail?id=2026
what should `sqrt(n).subs(n, 1)` be
sqrt(1), that is, 1. subs should not make any assumptions about mathematical equality of old and new. The reason is that it doesn't necessarily have to hold that old is the same as new in any sense. All we are doing is replacing ld with new in the expression. So while subs might make varying degrees of intelligent assumptions about how different parts of the expression are equal to old (like x**2 == (x**3)**(2/3)), but I don't think it should ever care what new even is. An exception to this might be something like (2*x).subs(x, Matrix(...)).
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