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