static S quote(O o) {
  if (o == null) return "null";
  ret quote(str(o));
}

static S quote(S s) {
  if (s == null) return "null";
  new StringBuilder out;
  quote_impl(s, out);
  ret out.toString();
}
  
static void quote_impl(S s, StringBuilder out) {
  out.append('"');
  int l = s.length();
  for (int i = 0; i < l; i++) {
    char c = s.charAt(i);
    if (c == '\\' || c == '"')
      out.append('\\').append(c);
    else if (c == '\r')
      out.append("\\r");
    else if (c == '\n')
      out.append("\\n");
    else
      out.append(c);
  }
  out.append('"');
}