patch applied (ghc): More fixes to pre-matching and pre-subsumption

[Simon Peyton Jones]

Tue Aug  8 15:49:24 PDT 2006  simonpj at microsoft.com
  * 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.
  

    M ./compiler/typecheck/TcUnify.lhs -20 +26