static <A> int nfilter(Iterable<A> c, IF1<A, Bool> pred) {
  ret nfilter(pred, c);
}

static <A> int nfilter(IF1<A, Bool> pred, Iterable<A> c) {
  int n = 0;
  if (c != null) for (A o : c)
    if (pred.get(o))
      ++n;
  ret n;
}

static int nfilter(Iterable c, O pred) {
  int n = 0;
  if (c != null) for (O o : c)
    if (isTrue(callF(pred, o)))
      ++n;
  ret n;
}

static int nfilter(O pred, Iterable c) {
  ret nfilter(c, pred);
}