static Set<SymVar> ai_newEngine_freeVariables(O o1, DeepZipTools tools) {
  new LinkedHashSet<SymVar> set;
  // basically just a visitor (could be optimized to visit only
  tools.metaTransformer().transform(o -> { if (o instanceof SymVar) set.add(o); null; }, o1);
  ret set;
}