+matchGroupArity :: MatchGroup id -> Arity
+matchGroupArity (MatchGroup (match:matches) _)
+ = ASSERT( all ((== n_pats) . length . hsLMatchPats) matches )
+ -- Assertion just checks that all the matches have the same number of pats
+ n_pats
+ where
+ n_pats = length (hsLMatchPats match)
+