@Test
void mapToDoubleIfPresent() {

    final ToDoubleNullable<String> toDoubleNullable = instance
            .mapToDoubleIfPresent(i -> 1);
    assertNotNull(toDoubleNullable);
    assertEquals(1, toDoubleNullable.applyAsDouble("three"));
    assertNull(toDoubleNullable.apply(null));
}