implement CSE optimization in Grin.SSimplify
implement CSE optimization in Grin.SSimplify

darcs-hash:20070603042335-1a7c6-96a188a7303098fbdb4d1de64899fbf48efaef68

kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc,kirbyfan64/jhc