text: function SubtractionPage(mathEngine) {
    PageBase.call(this, mathEngine);
    this.operator = "-";
  }
  SubtractionPage.prototype = Object.create(PageBase.prototype);
  SubtractionPage.prototype.constructor = SubtractionPage;
  
  (function(page){
    if (window) {
      window.onload = function() {
        console.log("Window Loaded.");
        page.load();
      }.bind(page);
    }
  })(new SubtractionPage(new MathEngine()));
  
  // override for substraction specific logic
  SubtractionPage.prototype.generateOperands = function() {
    if (!this.mathEngine) return {};
    var op1 = this.mathEngine.generateOperand(50);
    var op2 = this.mathEngine.generateOperand(op1);
    return {op1: op1, op2: op2};
  }
  SubtractionPage.prototype.isCorrect = function(op1, op2, answer) {
    if (!this.mathEngine) return false;
    return this.mathEngine.checkDifference(op1, op2, answer);
  }