docs: function() {
                      var target = this.target, sup = this.sup, sub = this.sub, options = this.options;
                      sup.scale(options.zoom);
                      sub.scale(options.zoom);
                      var targetBox = target.getFixRenderBox();
                      if (EmptyExpression.isEmpty(sup) && EmptyExpression.isEmpty(sub)) {
                          return {
                              width: targetBox.width,
                              height: targetBox.height,
                              top: 0,
                              bottom: 0
                          };
                      } else {
                          // 上标
                          if (!EmptyExpression.isEmpty(sup) && EmptyExpression.isEmpty(sub)) {
                              return this.applyUp(target, sup);
                          } else if (EmptyExpression.isEmpty(sup) && !EmptyExpression.isEmpty(sub)) {
                              return this.applyDown(target, sub);
                          } else {
                              return this.applyUpDownScript(target, sup, sub);
                          }
                      }
                  }