+let lref a i = LRef (a, i)
+
+let cast a u t = Cast (a, u, t)
+
+let appl a u t = Appl (a, u, t)
+
+let bind a b t = Bind (a, b, t)
+
+let bind_abst a u t = Bind (a, Abst u, t)
+
+let bind_abbr a v t = Bind (a, Abbr v, t)
+
+let bind_void a t = Bind (a, Void, t)
+
+(* local environment handling functions *************************************)
+
+let empty = Null
+
+let push e c a b = Cons (e, c, a, b)
+
+let rec get i = function
+ | Null -> Null, Null, [], Void
+ | Cons (e, c, a, b) when i = 0 -> e, c, a, b
+ | Cons (e, _, _, _) -> get (pred i) e
+
+let get e i = get i e