More fixes to pre-matching and pre-subsumption
Actually this patch fixes two errors. one was a trivial typo in tha last patch (b_ty should be b_tau), which led to an infinite loop when compiling Data.Generic.Twins. Fixing that revealed a more serious error in the same function. I was sloppy about dealing robsutly with name-capture for nested quantifiers in pre-subsumption and pre-matching; and sure enough I got bitten. Sigh. I think it is right now.
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