code: public static String subtractSeq(String first, String second, String delimiter) {
      if (isEmpty(first)) { return ""; }
      if (isEmpty(second)) {
        StringBuilder builder = new StringBuilder();
        if (!first.startsWith(delimiter)) builder.append(delimiter).append(first);
        if (!first.endsWith(delimiter)) builder.append(first).append(delimiter);
        return builder.toString();
      }
      List<String> firstSeq = Arrays.asList(split(first, delimiter));
      List<String> secondSeq = Arrays.asList(split(second, delimiter));
      Collection<String> rs = CollectUtils.subtract(firstSeq, secondSeq);
      StringBuilder buf = new StringBuilder();
      for (final String ele : rs)
        buf.append(delimiter).append(ele);
      if (buf.length() > 0) buf.append(delimiter);
      return buf.toString();
    }
docs: 返回一个新的逗号相隔字符串，实现其中的单词a-b的功能. 新的字符串将以,开始,结束<br>