docs: function(operator) {
                      if (operator === undefined) {
                          return this;
                      }
                      if (this.operator) {
                          this.operator.remove();
                      }
                      this.operatorBox.addShape(operator);
                      this.operator = operator;
                      this.operator.setParentExpression(this);
                      // 表达式关联到操作符
                      operator.expression = this;
                      return this;
                  }