docs: function() {
                      var target = this.target, sup = this.sup, sub = this.sub;
                      if (EmptyExpression.isEmpty(sup) && EmptyExpression.isEmpty(sub)) {
                          var targetRectBox = target.getRenderBox(this.observer);
                          return {
                              width: targetRectBox.width,
                              height: targetRectBox.height,
                              top: 0,
                              bottom: 0
                          };
                      } else {
                          // 下标处理
                          if (EmptyExpression.isEmpty(sup) && !EmptyExpression.isEmpty(sub)) {
                              return this.applySideSub(target, sub);
                          } else if (!EmptyExpression.isEmpty(sup) && EmptyExpression.isEmpty(sub)) {
                              return this.applySideSuper(target, sup);
                          } else {
                              return this.applySideScript(target, sup, sub);
                          }
                      }
                  }