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;
  }
}