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

`map --> key` returns the current binding of `key` in `map`, or raises `Not_found`. Equivalent to `find key map`.

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

`map <-- (key, value)` returns a map containing the same bindings as `map`, plus a binding of `key` to `value`. If `key` was already bound in `map`, its previous binding disappears. Equivalent to `add key value map`.