@Test
 public void shouldFlatMapArbitrary() {
   final Arbitrary<Integer> arbitraryInt = size -> Gen.choose(-size, size);
   final Arbitrary<BinaryTree<Integer>> arbitraryTree =
       arbitraryInt.flatMap(i -> new ArbitraryBinaryTree(-i, i));
   assertThat(arbitraryTree.apply(0).apply(new Random())).isNotNull();
 }