static L<TripleRef<Symbol>> ai_tripleRefs(T3<Symbol> t) {
  ret ai_tripleRefs(t.a, t.b, t.c);
}

static L<TripleRef<Symbol>> ai_tripleRefs(Symbol a, Symbol b, Symbol c) {
  ret ai_tripleRefs(a, b, c, thoughtSpace());
}

static L<TripleRef<Symbol>> ai_tripleRefs(Symbol a, Symbol b, Symbol c, AbstractThoughtSpace ts) {
  int position = tripleIndexOf(a, b, c, symbol_dollarX());
  if (position < 0) fail("No $X in " + ai_renderTriple(a, b, c));
  ret ai_filterTripleRefs(position, a, b, c, ai_thoughtSpace_query(a, b, c, ts));
}