static void web_dropAllNodesBut(WebNode... nodes) {
  web_dropAllNodesBut(first(nodes).web, asHashSet(nodes));
}
  
static void web_dropAllNodesBut(Web web, Set<WebNode> set) {
  for (WebNode node : web_nonRelationNodes(web))
    if (!set.contains(node))
      web_removeNode(node);
}