abstract sclass CTNode {

  new LinkedHashSet<CTNode> children;

  toString { ret joinNemptiesWithColonSpace(shortClassName(this), toString2()); }

  S toString2() { ret ""; } // override me

  bool add(CTNode n) { ret children.add(n); }

}

sclass ObjectNode<A> extends CTNode {

  A obj;

  *() {}

  *(A *obj) {}

  S toString2() { ret sfu(obj); }

  public bool equals(O o) { ret stdEq(this, o, 'obj); }

  public int hashCode() { ret stdHash(this, 'obj); }

}

sclass OpNode > CTNode {

  S op;

  bool haveResult;

  O result; // this is the result coming back up from the operation

  *() {}

  *(S *op) {}

  S toString2() { ret haveResult ? result + " <- " + op : op; }

  public bool equals(O o) { ret stdEq(this, o, 'op); }

  public int hashCode() { ret stdHash(this, 'op); }

  void setResult(O o) {

    result = o;

    haveResult = true;

  }

}