@Override
  public String toString() {
    return "FileTrees{" +
           "stub=" + (myStub == null ? "noRef" : derefStub()) +
           ", AST=" + (myTreeElementPointer == null ? "noRef" : derefTreeElement()) +
           ", useSpineRefs=" + useSpineRefs() +
           '}' ;
  }
@Override
  public String toString()