public function mapWithKey( callable $callback, ): ImmSet<Tu>
ImmSets don't have keys, the callback uses the values as the keys
Every value in the current
ImmSet is affected by a call to
filterWithKey() where only values that meet a
certain criteria are affected.
callable $callback- The callback containing the operation to apply to the current
ImmSetkeys and values.
ImmSetcontaining the values after a user-specified operation on the current
ImmSet's values is applied.
Set::mapWithKey for usage examples.