MutableSet containing the values after an operation has been
applied to each "key" and value in the current Set
public function mapWithKey<Tu>( (function(mixed,Tv):Tu) $fn, ): MutableSet<Tu>;
Since sets don't have keys, the callback uses the values as the keys as well.
(function(mixed,Tv):Tu) $fn- The callback containing the operation to apply to the current
MutableSet"keys" and values.