Xpath to select only nodes where child elements exist?

Question

This should be an easy one but it is giving me trouble. Given this structure:

<root>
  <a>
    <b/>
  </a>
  <a/>
</root>

I'm trying to formulate an xpath expression that gives only the non-empty "a" elements, i.e. the ones that have child elements. Therefore I want the first instance of "a" returned, but not the second.

So far I have "/root/a/self::*" but that is returning me both a's.

Answer 1

/root/a[count(*)>0]

will give any 'a' node with any kind of child node

Answer 2

/root/a[count(*)>0]