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The before/after delta is very satisfying and clearly easier to work with. Very nice.

How does the DSL handle side effects? I'm assuming any term here could be a compound expression, so what part of the system prevents "0*callfn()" from reducing to "0"?

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I believe this operates on values in something akin to registers, not expression trees. By the time you get to a place where one of these rules is relevant, the side effect has already taken place. This just prevents the return of such a function call to be operated on, like in your example.

Yeah, that's exactly right. It's operating on an SSA-based intermediate representation. So if an operation gets removed, the arguments are still being computed. The earlier operations that produce those arguments can also be removed by dead code elimination (which runs later), but indeed only if they have no side effects.

I suppose the notation of the rule heads makes it look like there actually are expression trees, which is maybe sort of confusing.

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