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.