sclass DeepZipTools {
  // define what a variable is
  swappable bool isVar(O o) {
    ret o instanceof S && isDollarVar(o/S)
      || o instanceof SymVar; }
  
  // and the usual transformation utils
  swappable Map deepZip(O a, O b) { ret newGazelle_deepZip1(a, b); }
  swappable O replaceVars(O o, Map map) { ret replaceVars_understandsTransformableAndList(o, map); }
}