static <A extends Concept> Cl<A> cForwardRefs(Concept c, Class<A> type) {
  ret c == null ?: instancesOf(type, cForwardRefs(c));
}
  
static <A extends Concept> Cl<A> nuLike cForwardRefs(Class<A> type, Concept c) {
  ret cForwardRefs(c, type);
}

static Cl<Concept> cForwardRefs(Concept c) {
  ret c == null ?: lmap derefRef(c._refs());
}