docs: public SimplifySpanBuild append(BaseSpecialUnit specialUnit) {
          if (null == specialUnit) return this;
  
          String specialText = specialUnit.getText();
          if (TextUtils.isEmpty(specialText)) return this;
  
          // process start pos
          specialUnit.setStartPoss(new int[]{mStringBuilder.length()});
          mStringBuilder.append(specialText);
          mFinalSpecialUnit.add(specialUnit);
          return this;
      }