// filter: S -> bool
static void printExplainTree2_filterClasses(Explain e, O filter) {
  printExplainTree2_filterClasses(0, e, filter);
}

static void printExplainTree2_filterClasses(int indent, Explain e, O filter) {
  bool printNulls = false;
  
  if (e == null) {
    // this would print "null" for every <identifier> etc
    if (printNulls)
      print(indent(indent) + "null");
  } else {
    new L<S> classNames;
    while true {
      addIfMatchesPredicate(classNames, e.className(), filter);
      if (!e.singleEqualChild()) break;
      e = e.sub(0);
    }
    print(indent(indent) + join(", ", classNames) + " | " + e.string());
    for (Explain sub : e.subs)
      printExplainTree2_filterClasses(indent+2, sub, filter);
  }
}