`val (-->) : ('a, 'b) t -> 'a -> 'b`

`tbl-->x` returns the current binding of `x` in `tbl`, or raises `Not_found` if no such binding exists. Equivalent to `Hashtbl.find tbl x`

`val (<--) : ('a, 'b) t -> ('a * 'b) -> unit`

`tbl<--(x, y)` adds a binding of `x` to `y` in table `tbl`. Previous bindings for `x` are not removed, but simply hidden. That is, after performing `Hashtbl.remove` `tbl x`, the previous binding for `x`, if any, is restored. (Same behavior as with association lists.) Equivalent to `Hashtbl.add tbl x y`