### Question:
IsNullImpl implements IsNull<T, R> { @Override public ToNullable<T, R, ?> expression() { return expression; } IsNullImpl(ToNullable<T, R, ?> expression); @Override IsNotNull<T, R> negate(); @Override ToNullable<T, R, ?> expression();  }### Answer:
@Test void expression() { assertNotNull(instance.expression()); }