func TestWriterReadFromCounts(t *testing.T) {
	var w0 writeCountingDiscard
	b0 := NewWriterSize(&w0, 1234)
	b0.WriteString(strings.Repeat("x", 1000))
	if w0 != 0 {
		t.Fatalf("write 1000 'x's: got %d writes, want 0", w0)
	}
	b0.WriteString(strings.Repeat("x", 200))
	if w0 != 0 {
		t.Fatalf("write 1200 'x's: got %d writes, want 0", w0)
	}
	io.Copy(b0, onlyReader{strings.NewReader(strings.Repeat("x", 30))})
	if w0 != 0 {

   * products that you would get from nesting for loops:
   *
   * {@snippet :
   * for (B b0 : sets.get(0)) {
   *   for (B b1 : sets.get(1)) {
   *     ...
   *     ImmutableList<B> tuple = ImmutableList.of(b0, b1, ...);
   *     // operate on tuple
   *   }
   * }
   * }
   *