docs: function BackButton(options) {
          var that;
          that = this;
  
          /**
           * ### BackButton.button
           *
           * The HTML element.
           */
          if ('object' === typeof options.button) {
              this.button = options.button;
          }
          else if ('undefined' === typeof options.button) {
              this.button = document.createElement('input');
              this.button.type = 'button';
          }
          else {
              throw new TypeError('BackButton constructor: options.button must ' +
                                  'be object or undefined. Found: ' +
                                  options.button);
          }
  
          /**
           * ### BackButton.acrossStages
           *
           * If TRUE, it allows to go back to previous stages
           *
           * Default: FALSE
           */
          this.acrossStages = null;
  
          /**
           * ### BackButton.acrossRounds
           *
           * If TRUE, it allows to go back previous rounds in the same stage
           *
           * Default: TRUE
           */
          this.acrossRounds = null;
  
          this.button.onclick = function() {
              var res;
              res = getPreviousStep(that);
              if (!res) return;
              res = node.game.gotoStep(res);
              if (res) that.disable();
          };
      }