code: public static Ix<Integer> characters(CharSequence cs, int start, int end) {
          int len = cs.length();
          if (start < 0 || end < 0 || start > len || end > len) {
              throw new IndexOutOfBoundsException("start=" + start + ", end=" + end + ", length=" + len);
          }
          return new IxCharacters(cs, start, end);
      }
docs: Emits a range of characters from the given CharSequence as integer values.
  <p>
  The result's iterator() doesn't support remove().
  @param cs the source character sequence, not null
  @param start the start character index, inclusive, non-negative
  @param end the end character index, exclusive, non-negative
  @return the new Ix instance
  @throws NullPointerException if cs is null
  @throws IndexOutOfBoundsException if start is out of range [0, cs.length]
  @since 1.0