static CharacterClassIterator characterClassIteratorFromMiniRegexp_v1(S regexp, int i default 0) {

  int l = l(regexp);

  if (i >= l) ret new CCI_End;

  char c = regexp.charAt(i);

  // backslash (escaped character)

  if (c == '\\' && i+1 < l)

    ret CCI_SingleChar(regexp.charAt(i+1),

      characterClassIteratorFromMiniRegexp_v1(regexp, i+2));

  // optional character

  if (i+1 < l && regexp.charAt(i+1) == '?') {

    CharacterClassIterator rest = characterClassIteratorFromMiniRegexp_v1(regexp, i+2);

    // TODO: coalesce choice objects when rest is already a choice?

    new HashMap<Char, CharacterClassIterator> map; // TODO: optional comparator

    fOr (char cc : rest.acceptedChars())

      map.put(cc, rest);

    // may override an assignment made above

    map.put(c, new CCI_SingleChar(c, rest));

    ret CCI_Choice(rest.canEndHere(), map);

  }

  // normal character

  ret CCI_SingleChar(regexp.charAt(i),

    characterClassIteratorFromMiniRegexp_v1(regexp, i+1));

}