static MultiMap<S, WebNode> ai_indexWebs(Collection<Web> webs) {
  if (hasMultiCore() && l(webs) >= 2000) ret ai_indexWebs_parallel2(webs);
  ret ai_indexWebs_serial(webs);
}
    
static MultiMap<S, WebNode> ai_indexWebs_serial(Collection<Web> webs) {
  MultiMap<S, WebNode> index = ai_emptyWebIndex();
  for (Web web : unnull(webs))
    ai_addWebToIndex(index, web);
  ret index;
}