static S stringToLegalIdentifier(S s) {
  new StringBuilder buf;
  s = dropTrailingSquareBracketStuff(s);
  for i over s: {
    char c = s.charAt(i);
    if (empty(buf) ? Character.isJavaIdentifierStart(c) : Character.isJavaIdentifierPart(c))
      buf.append(c);
  }
  if (empty(buf)) fail("Can't convert to legal identifier: " + s);
  ret str(buf);
}