static S ai_subst_web(S webID) {
  int n = websMadeInThread();
  Web web = allWebsByID().get(webID);
  if (web == null)
    ret "Web not found: " + webID;
  WebRelation rel = web_findRelationNamed(web, "matches");
  if (rel == null) ret "No 'matches' relation";
  S pat = web_unquoted(rel.a), input = web_unquoted(rel.b);
  if (neq(pat, "substitute * => * with *"))
    ret "Wrong pattern";
  new Matches m;
  if (!flexMatchIC(pattern, input, m))
    ret "No match";
  S src = unquote(m.get(0));
  L<S> terms = unquoteAll(trimAll(splitAtJavaToken(m.get(1), ",")));
  for (S term : terms)
    ai_postTriple(src, "see:", term);
  ret nWebs(websMadeInThread()-n) + " made";
}