docs: public SimplifySpanBuild appendToFirst(BaseSpecialUnit specialUnit) {
          if (null == specialUnit) return this;
  
          String specialText = specialUnit.getText();
          if (TextUtils.isEmpty(specialText)) return this;
  
          // process start pos
          int curBeforeFirstPos = mBeforeStringBuilder.length();
          specialUnit.setStartPoss(new int[]{curBeforeFirstPos});
          mBeforeStringBuilder.insert(curBeforeFirstPos, specialText);
          mBeforeSpecialUnit.add(specialUnit);
          return this;
      }