   }
   
   func testPushesElements() {
-    var queue: PriorityQueue<Int> = []
+    var queue = PriorityQueue<Int>()
     
     queue.enqueue(1)
     queue.enqueue(2)

  }
  
  func testPushesElement() {
    var queue: PriorityQueue<Int> = []
    
    queue.enqueue(1)
    XCTAssertFalse(queue.isEmpty)

    XCTAssertEqual(nil, queue.front)
  }
  
  func testInitialisesFromEmptySequenceWithNaturalOrdering() {
    let queue = PriorityQueue<Int>([], isOrdered: <)
    
    XCTAssertTrue(queue.isEmpty)
    XCTAssertEqual(0, queue.count)
    XCTAssertEqual(nil, queue.front)
  }
  
  func testInitialisesFromEmptySequenceWithNonNaturalOrdering() {
    let queue = PriorityQueue<Int>([], isOrdered: >)
    
    XCTAssertTrue(queue.isEmpty)
    XCTAssertEqual(0, queue.count)
    XCTAssertEqual(nil, queue.front)
  }
  
  func testInitialisesFromSequence() {
    let queue = PriorityQueue<Int>([1, 2, 3])
    
