+exception NotSimilar
+let alpha t1 t2 ref ref' =
+ let rec aux t1 t2 = match t1,t2 with
+ | NCic.Rel n, NCic.Rel m when n=m -> ()
+ | NCic.Appl l1, NCic.Appl l2 -> List.iter2 aux l1 l2
+ | NCic.Lambda (_,s1,t1), NCic.Lambda (_,s2,t2)
+ | NCic.Prod (_,s1,t1), NCic.Prod (_,s2,t2) -> aux s1 s2; aux t1 t2
+ | NCic.LetIn (_,s1,ty1,t1), NCic.LetIn (_,s2,ty2,t2) ->
+ aux s1 s2; aux ty1 ty2; aux t1 t2
+ | NCic.Const r1,
+ NCic.Const r2 when NReference.eq r1 ref && NReference.eq r2 ref' -> ()
+ | NCic.Const r1, NCic.Const r2 when NReference.eq r1 r2 -> ()
+ | NCic.Meta _,NCic.Meta _ -> ()
+ | NCic.Implicit _,NCic.Implicit _ -> ()
+ | NCic.Sort x,NCic.Sort y when x=y -> ()
+ | NCic.Match (_,t1,t11,tl1), NCic.Match (_,t2,t22,tl2) ->
+ aux t1 t2;aux t11 t22;List.iter2 aux tl1 tl2
+ | _-> raise NotSimilar
+ in
+ try aux t1 t2; true with NotSimilar -> false
+;;
+