public function testAllLeavesStatic()
 {
     $allLeaves = Cluster::allLeaves()->get();
     $this->assertCount(4, $allLeaves);
     $leaves = $allLeaves->lists('name');
     $this->assertContains('Child 1', $leaves);
     $this->assertContains('Child 2.1', $leaves);
     $this->assertContains('Child 3', $leaves);
     $this->assertContains('Root 2', $leaves);
 }