A transform pass done before optimization can begin to fix up some oddities in the tree output by the stack checker:

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We rewrite the code so that all #introduce nodes are replaced with a single one, at the beginning of a program. This simplifies subsequent analysis.

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We normalize #call-recursive as follows. The stack checker says that the inputs of a #call-recursive are the entire stack at the time of the call. This is a conservative estimate; we don't know the exact number of stack values it touches until the #return-recursive node has been visited, because of row polymorphism. So in the normalize pass, we split a #call-recursive into a #copy of the unchanged values and a #call-recursive with trimmed inputs and outputs.