sS dropSpaceEtc(S s) {
  elegant {
    ret filterChars(c -> !isSpaceEtc(c), s);
  } inlined {
    replace test with !isSpaceEtc .
    
    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 : "";
  }
}