Is that kind of structure allowed in XML?

Is that kind of structure allowed in XML ?

<category>
    <pattern>ANSWER MY QUESTION</pattern>
    <template>
        Please try asking 
        <set name="it">y开发者_运维百科our question</set> 
        another way.
    </template>   
</category>

Yes, it's perfectly valid. In XML Schema parlance, it's known as the "mixed content" model.

Whether or not it's a good idea, however, is another question entirely.

Why wouldn't it be? XML can be anything you as long as it's properly formatted, and that looks properly formatted as far as I'm concerned.