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
$callback- The callback containing the condition to apply to the current
ImmSetkeys and values.
ImmSetcontaining the values after a user-specified condition is applied to the values of the current
Set::filterWithKey for usage examples.