docs: function(operand, index, isWrap) {
                      // 不包装操作数
                      if (isWrap === false) {
                          this.operands[index] = operand;
                          return this;
                      }
                      operand = Expression.wrap(operand);
                      if (this.operands[index]) {
                          this.operands[index].remove();
                      }
                      this.operands[index] = operand;
                      this.operandBox.addShape(operand);
                      return this;
                  }