HH\Set::mapWithKey
Returns a Set containing the values after an operation has been applied
to each "key" and value in the current Set
public function mapWithKey<Tu as arraykey>(
(function(arraykey, Tv): Tu) $callback,
): Set<Tu>;
Since Sets don't have keys, the callback uses the values as the keys
as well.
Every value in the current Set is affected by a call to mapWithKey(),
unlike filterWithKey() where only values that meet a certain criteria are
affected.
Parameters
(function(arraykey, Tv): Tu) $callback