Phase control

[ayazhafiz]

It would be cool if our rules also supported phase control, like ghc does (https://downloads.haskell.org/~ghc/7.0.1/docs/html/users_guide/rewrite-rules.html). The idea is we provide some pragma for string rules specifying what phase they run in. For example

{2} a + b -> b + a

would only be run in the second "phase" of term rewriting. How this fits in with more powerful optimization and how we decide which internal rules are in which phase I haven't quite figured out yet.

An extension of this is auto-placement of rules. If a rule has no phase pragma, some heuristic or lightweight pass is run on it to figure out which other rules it most directly affects. Not sure if this even works, but could be something for the ML ideas we were thinking about @luke-bhan.