Set containing the values after an operation has been applied
to each "key" and value in the current
public function mapWithKey<Tu as arraykey>(
(function(arraykey, Tv): Tu) $callback,
Sets don't have keys, the callback uses the values as the keys
Every value in the current
Set is affected by a call to
filterWithKey() where only values that meet a certain criteria are
(function(arraykey, Tv): Tu) $callback - The callback containing the operation to apply to the
Set keys and values.
Set<Tu> - a
Set containing the values after a user-specified operation
on the current
Set's values is applied.