code: protected char selectFlag(final String flags, final char[] choice) {
          if (flags == null) return 0;
          StringBuilder conflict = null;
          char found = 0;
          for (int i = 0; i < flags.length(); i++) {
              char f = flags.charAt(i);
              if (oneOf(f, choice)) {
                  if (found != 0) {
                      if (conflict == null) conflict = new StringBuilder().append(found);
                      conflict.append(f);
                  } else {
                      found = f;
                  }
              }
          }
          if (conflict != null) {
              throw FormatException.illegalFlags(
                  conflict.toString(), getFormatName());
          }
          return found;
      }
docs: Selects exactly one of the expected flags.
  @param choice a {@link #flags(java.lang.String) flags} array
  @param flags
  @return one of {@code choice}, or {@code '\0'}
  @throws FormatFlagsConversionMismatchException if more than one expected flag was found.