improve speed of subst by using fromAscList
improve speed of subst by using fromAscList

darcs-hash:20050920013905-1a7c6-fbae8ceb203da7d215c29601f5a26b407758ad21

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