sS lambdaMapLike filterChars(IPred<Char> pred, S s) {
  replace test with pred.get .
  
  if (empty(s)) null;
  int n = l(s), i = 0;
  char c = s.charAt(i);
  bool totality = test(c);
  for (++i; i < n; i++) {
    c = s.charAt(i);
    bool b = test(c);
    
    if (b != totality) {
      // diverse case
      new StringBuilder buf;
      if (totality) buf.append(substring(s, 0, i));
      while ping (true) {
        if (b) buf.append(c);
        if (++i >= n) break;
        c = s.charAt(i);
        b = test(c);
      }
      ret buf.toString();
    }
  }
    
  ret totality ? s : "";
}