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.');
}