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