The type of your datas' keys.
type 'a t
The type of your data structure with 'a-typed values.
val fold : ( key -> 'a -> 'b -> 'b ) -> 'a t -> 'b -> 'b
A fold function on your data structure. Its role is to apply a function to every binding of your structure, building the Hamt by accumulation on the third parameter of this function.