public function testMacroable()
 {
     // Foo() macro : unique values starting with A
     Collection::macro('foo', function () {
         return $this->filter(function ($item) {
             return strpos($item, 'a') === 0;
         })->unique()->values();
     });
     $c = new Collection(['a', 'a', 'aa', 'aaa', 'bar']);
     $this->assertSame(['a', 'aa', 'aaa'], $c->foo()->all());
 }