-type instance IsCommonExprSet HNil = True
-type instance IsCommonExprSet (HCons e es) = IsCommonExpr e :&&: IsCommonExprSet es
+type instance IsCommonExprSet Nil = True
+type instance IsCommonExprSet (Cons e es) = IsCommonExpr e :&&: IsCommonExprSet es
+
+type family IsIterativeExprSet es
+type instance IsIterativeExprSet Nil = True
+type instance IsIterativeExprSet (Cons e es) = IsIterativeExpr e :&&: IsIterativeExprSet es