static DependentCache<MultiMap<GlobalID>> ai_trueWebs_cache = new DependentCache(f ai_trueWebs_calc, f cachedNodeIndex /*_changeCount*/);

static Set<GlobalID> ai_trueWebs() {
  ret keys(ai_trueWebs_cache!);
}

static MultiMap<GlobalID> ai_trueWebs_calc() {
  ai_onNewOrRemovedWeb(f ai_trueWebs_onNewWeb, f ai_trueWebs_onRemovedWeb);
  ai_trueWebs_cache.set(new MultiMap);
  Collection<Web> webs = allWebsFromCachedNodeIndex_uncached();
  for (Web web : webs)
    ai_trueWebs_onNewWeb(web);
  ret ai_trueWebs_cache!;
}

svoid ai_trueWebs_onNewWeb(Web web) {
  if (web.unverified) ret;
  lock ai_trueWebs_cache.lock;
  new Matches m;
  for (WebNode n : web_search_dollarX(webFromTriple("$X", "is", "true"), web))
    if (web_match("Web *", n, m) && isGlobalID($1))
      ai_trueWebs_cache->put(asGlobalID($1), web.globalIDObj());
}

svoid ai_trueWebs_onRemovedWeb(Web web) {
  if (web.unverified) ret;
  lock ai_trueWebs_cache.lock;
  new Matches m;
  for (WebNode n : web_search_dollarX(webFromTriple("$X", "is", "true"), web))
    if (web_match("Web *", n, m) && isGlobalID($1))
      ai_trueWebs_cache->remove(asGlobalID($1), web.globalIDObj());
}

Began life as a copy of #1011416