code: public void back() throws JSONException {
          if (this.usePrevious || this.index <= 0) {
              throw new JSONException("Stepping back two steps is not supported");
          }
          this.decrementIndexes();
          this.usePrevious = true;
          this.eof = false;
      }
docs: Back up one character. This provides a sort of lookahead capability,
  so that you can test for a digit or letter before attempting to parse
  the next number or identifier.
  @throws JSONException Thrown if trying to step back more than 1 step
  or if already at the start of the string