(PECL ds >= 1.2.7)
Ds\Set::map — Returns the result of applying a callback to each value
   Returns the result of applying a callback function to
   each value in the set.
  
callbackThe callback to apply to each value in the set must have the following signature:
   Returns a new Ds\Set instance where each value
   is the result of applying the callback to each value
   of the set.
  
例1 Ds\Set::map() example
<?php
$set = new \Ds\Set([1, 2, 3]);
var_dump($set->map(function($value) { return $value * 2; }));
var_dump($set);
?>上の例の出力は、 たとえば以下のようになります。
object(Ds\Set)#3 (3) {
  [0]=>
  int(2)
  [1]=>
  int(4)
  [2]=>
  int(6)
}
object(Ds\Set)#1 (3) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
}
