public function filterWithKey( (function(mixed,Tv):bool) $callback, ): ImmSet<Tv>;
ImmSets don't have keys, the callback uses the values as the keys
Only values that meet a certain criteria are affected by a call to
filterWithKey(), while all values are affected by a call to
(function(mixed,Tv):bool) $callback- The callback containing the condition to apply to the current
ImmSetkeys and values.