sig lists. kind pairty type -> type -> type. type pair A -> B -> (pairty A B). type id (list A) -> (list A) -> o. type memb A -> (list A) -> o. type member A -> (list A) -> o. type append (list A) -> (list A) -> (list A) -> o. type join (list A) -> (list A) -> (list A) -> o. type assoc A -> B -> (list (pairty A B)) -> o. type domain (list (pairty A B)) -> (list A) -> o. type range (list (pairty A B)) -> (list B) -> o.