1signature Subst = 2sig 3 eqtype 'a subs 4 5 val id : 'a subs 6 val cons : 'a subs * 'a -> 'a subs 7 val shift : int * 'a subs -> 'a subs 8 val lift : int * 'a subs -> 'a subs 9 val is_id : 'a subs -> bool 10 val exp_rel : 'a subs * int -> int * 'a option 11 val comp : ('a subs * 'a -> 'a) -> 'a subs * 'a subs -> 'a subs 12 13end 14