public function testOnce()
 {
     // from js + docs
     $num = 0;
     $increment = __u::once(function () use(&$num) {
         return $num++;
     });
     $increment();
     $increment();
     $this->assertEquals(1, $num);
     // extra
     $num = 0;
     $increment = __u(function () use(&$num) {
         return $num++;
     })->once();
     $increment();
     $increment();
     $this->assertEquals(1, $num);
 }