public function testGetLeftTree()
 {
     $obj_binaryTree = new BinaryTree('content', new BinaryTree('content L'), new BinaryTree('content R'));
     $this->assertEquals('content L', $obj_binaryTree->getLeftTree()->getObject(), 'Left tree\'s content should equal content L.');
     $obj_binaryTreeEmpty = new BinaryTree();
     $this->assertNull($obj_binaryTreeEmpty->getLeftTree(), 'Left tree should be null.');
 }