Comment #5 on issue 1908 by [email protected]: mutable core
http://code.google.com/p/sympy/issues/detail?id=1908

Of course, an efficient implementation has to use mutable structures internally. But this is quite unrelated to the mutability of the object itself. To get an "immutable" object, you only need a __hash__() and to ensure that the equality relation is constant wrt. time (i.e. if a == b at one time, then it is true at all times).

IIRC, the sympycore solution is to consider objects initially mutable until their
hash is computed, and immutable afterwards.

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