Set nillable element with Relax NG

Is there a way to declare that an element can be null using Relax NG, something like xsi:nillable="true" using XS开发者_StackOverflowD?

According to J. Clark in The Design of Relax NG Null values are not included as part of Relax NG. He proposes some alternatives to deal with these cases but we can argue that some times (i.e. array programming) the Null values are necessary as place holders. If we define extra arguments or elements to show that an element is null, we increase the complexity of the element structure though.