get_lit (LitPat lit) = Just lit
get_lit (NPat (HsIntegral i _) mb _ _) = Just (HsIntPrim (mb_neg mb i))
get_lit (NPat (HsFractional f _) mb _ _) = Just (HsFloatPrim (mb_neg mb f))
+get_lit (NPat (HsIsString s _) _ _ _) = Just (HsStringPrim s)
get_lit other_pat = Nothing
mb_neg :: Num a => Maybe b -> a -> a