static L<TripleRef<Symbol>> ai_checkTripleRefs(Symbol a, Symbol b, Symbol c, L<TripleRef<Symbol>> l) {
  if (empty(l)) ret sharedEmptyList();
  new L<TripleRef<Symbol>> out;
  for (TripleRef<Symbol> ref : l) if (ref != null)
    if (canSimpleMatchTriple_dollarVars(a, b, c, ref.triple))
      out.add(ref);
  ret out;
}