code: public final T substr(int beginIndex) {
          if (beginIndex < 0) {
              throw new StringIndexOutOfBoundsException(beginIndex);
          }
          return (beginIndex == 0) ? me() : substr(beginIndex, size());
      }
docs: Synonym of {@link #substring(int)} but return Str instead of String
  @param beginIndex the begin index
  @return A sub str that starts from the index specified