public function testIsLeaf()
 {
     $this->assertTrue($this->clusters('Child 1')->isLeaf());
     $this->assertTrue($this->clusters('Child 2.1')->isLeaf());
     $this->assertTrue($this->clusters('Child 3')->isLeaf());
     $this->assertTrue($this->clusters('Root 2')->isLeaf());
     $this->assertFalse($this->clusters('Root 1')->isLeaf());
     $this->assertFalse($this->clusters('Child 2')->isLeaf());
     $new = new Cluster();
     $this->assertFalse($new->isLeaf());
 }