###First the code fragment...

```
<tag name="default" abc="10" def="20"> <!-- not valid, abc and def should be mutually exclusive -->

<tag name="default1" abc="10"> <!-- valid -->

<tag name="default2" def="20"> <!-- valid -->

```

###What I want to do...

What can I put into my `XSD` so that `@abc` and `@def` cannot coexist as attributes on the same element?

So that validation would fail if they coexisted on the same element?