static ai_spec_gc_clear(S word) {
  int n = 0;
  for (WebRelation rel : indexedRelations("has GC distance"))
    if (ai_isTripleWeb(rel.web)) {
      ai_invalidateWeb(rel.web);
      ++n;
    }
  if (n != 0) print("Cleared " + n(n, "GC entries"));
}