The _push() and _pop() methods aren't really any different from
    the push() and pop() methods from the base MultiMap, so we can use
    them directly to avoid a lot of looks and method invocation
    overhead, and it's easier to tell from the code that they are just
    a renaming.

Context.beginScope(), .endScope():
    Move an attribute access out of the loop since the attribute isn't
    re-bound inside the loop.

Context.evaluateText():
    Re-phrase the condition in the if statement to allow the runtime
    to perform fewer global name lookups when possible, and avoid the
    tuple construction completely.