static S xyzToNewIDs(S s, L<S> varCollector) {
  L<S> tok = javaTok(s);
  int n = numberOfXYZVars(tok);
  for (int i = 1; i <= n; i++) {
    S var = xyzIndexToVar(i);
    if (tok.contains(var))
      replace(tok, var, addAndReturn(varCollector, aGlobalID()));
  }
  ret join(tok);
}