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));
}