docs: function calculatorDirective() {
  
      return {
          controller: Controller,
          controllerAs: 'ctrl',
          restrict: 'EA',
          scope: {},
          template: HTML
      };
  
      function Controller() {
          this.operand1 = null;
          this.operand2 = null;
          this.result = null;
  
          this.add = function() {
              this.result = parseInt(this.operand1) + parseInt(this.operand2);
          };
  
          this.isAddDisabled = function() {
              return isNaN(parseInt(this.operand1)) || isNaN(parseInt(this.operand2));
          };
      }
  }