`val add : key:key -> data:'a -> 'a t -> 'a t`
`val iter : f:(key:key -> data:'a -> unit) -> 'a t -> unit`
`val map : f:('a -> 'b) -> 'a t -> 'b t`
`val mapi : f:(key:key -> data:'a -> 'b) -> 'a t -> 'b t`
`val filterv : f:('a -> bool) -> 'a t -> 'a t`
`val filter : f:(key -> 'a -> bool) -> 'a t -> 'a t`
`val fold : f:(key:key -> data:'a -> 'b -> 'b) -> 'a t -> init:'b -> 'b`
`val compare : cmp:('a -> 'a -> int) -> 'a t -> 'a t -> int`
`val equal : cmp:('a -> 'a -> bool) -> 'a t -> 'a t -> bool`