+ val merge : merge_conf -> elt -> t -> t -> t
+ val mk_quick_tag_loop : (elt -> elt -> 'a*t array) -> 'a -> int -> Tree.t -> Tag.t -> (elt -> elt -> 'a*t array)
+ val mk_quick_star_loop : (elt -> elt -> 'a*t array) -> 'a -> int -> Tree.t -> (elt -> elt -> 'a*t array)