// testing Stack
"A Stack" should "pop values in last-in-first-out order" in {
    val stack = new Stack[Int]
    stack.push(1)
    stack.push(2)
    assert(stack.pop() === 2)
    assert(stack.pop() === 1)
}

// testing collections
List(1, 2, 2, 3, 3, 3) should contain theSameElementsAs Vector(3, 2, 3, 1, 2, 3)