+exception Subst_not_found of int
+
+let lookup_subst n subst =
+ try
+ List.assoc n subst
+ with Not_found -> raise (Subst_not_found n)
+
+let fold f g acc = function
+ | NCic.Implicit _
+ | NCic.Sort _
+ | NCic.Const _
+ | NCic.Meta _
+ | NCic.Rel _ -> acc
+ | NCic.Appl l -> List.fold_left f acc l
+ | NCic.Prod (_,s,t)
+ | NCic.Lambda (_,s,t) -> f (g (f acc s)) t
+ | NCic.LetIn (_,ty,t,bo) -> f (g (f (f acc ty) t)) bo
+ | NCic.Match (_,oty,t,pl) -> List.fold_left f (f (f acc oty) t) pl
+;;
+
+let sharing_map f l =
+ let unchanged = ref true in
+ let rec aux b = function
+ | [] as t -> unchanged := b; t
+ | he::tl ->
+ let he1 = f he in
+ he1 :: aux (b && he1 == he) tl
+ in
+ let l1 = aux true l in
+ if !unchanged then l else l1
+;;
+
+let map f g k = function
+ | NCic.Implicit _
+ | NCic.Sort _
+ | NCic.Const _
+ | NCic.Meta _
+ | NCic.Rel _ as t -> t
+ | NCic.Appl l -> NCic.Appl (sharing_map (f k) l)
+ | NCic.Prod (n,s,t) as orig ->
+ let s1 = f k s in let t1 = f (g k) t in
+ if t1 == t && s1 == s then orig else NCic.Prod (n,s1,t1)
+ | NCic.Lambda (n,s,t) as orig ->
+ let s1 = f k s in let t1 = f (g k) t in
+ if t1 == t && s1 == s then orig else NCic.Prod (n,s1,t1)
+ | NCic.LetIn (n,ty,t,b) as orig ->
+ let ty1 = f k ty in let t1 = f k t in let b1 = f (g k) b in
+ if ty1 == ty && t1 == t && b1 == b then orig else NCic.LetIn (n,ty1,t1,b1)
+ | NCic.Match (r,oty,t,pl) as orig ->
+ let oty1 = f k oty in let t1 = f k t in let pl1 = sharing_map (f k) pl in
+ if oty1 == oty && t1 == t && pl1 == pl then orig
+ else NCic.Match(r,oty1,t1,pl1)
+;;
+