Qed.IntsetSet of integers using Patricia Trees.
From the paper of Chris Okasaki and Andrew Gill: 'Fast Mergeable Integer Maps'.
val empty : tval singleton : int -> tval is_empty : t -> boolval cardinal : t -> intval elements : t -> int listval mem : int -> t -> boolval iter : ( int -> unit ) -> t -> unitval fold : ( int -> 'a -> 'a ) -> t -> 'a -> 'aval for_all : ( int -> bool ) -> t -> boolval exists : ( int -> bool ) -> t -> bool