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In underscore.js's original implementation of eq, "return false" implies
that isEqual would be false, and the method would short-circuit, which
made it unnecessary to maintain the proper state of the aStack/bStack.

With isSimilar, this undocumented assumption is no longer true, because
eq() may be used recursively (in particular: within toSet), so we have
to correctly maintain the stack upon return, by popping the pushed item
atop the stack.