NS_NOTYETIMPLEMENTED("write me!"); return NS_ERROR_NOT_IMPLEMENTED;
NS_PRECONDITION(aPreviousSibling != nsnull, "null ptr"); if (! aPreviousSibling) return NS_ERROR_NULL_POINTER;  *aPreviousSibling = nsnull; return NS_OK;
XULDocumentImpl::GetPreviousSibling(nsIDOMNode** aPreviousSibling){    NS_NOTYETIMPLEMENTED("write me!");    return NS_ERROR_NOT_IMPLEMENTED;}