And in Naf does not validate in dr.xsd, but wrapped in Operator it does

This does not validate:

```
<ruleml:Naf xmlns:ruleml="http://ruleml.org/spec">
        <ruleml:And/>
</ruleml:Naf>
```

but this does:

```
<ruleml:Naf xmlns:ruleml="http://ruleml.org/spec">
     <ruleml:Operation>
          <ruleml:And/>
     </ruleml:Operation>   
</ruleml:Naf>
```
