/* Stack data-structure. It's work is based on the LIFO method (last-IN-first-OUT).
 * It means that elements added to the stack are placed on the top and only the
 * last element (from the top) can be reached. After we get access to the last
 * element, he pops from the stack.
 * This is a class-based implementation of a Stack. It provides functions
 * 'push' - to add an element, 'pop' - to remove an element from the top.
 * Also it implements 'length', 'last' and 'isEmpty' properties and
 * static isStack method to check is an object the instance of Stack class.
 */

// Class declaration
class Stack {
  constructor () {
    this.stack = []
    this