[ZPT] concatenating strings in tal:attributes?

[Adam]

I would like to do something like this:
<a href="URL" tal:attributes="href folders/absolute_url + '?content=' + 
docs/id">blah</a>

The idea being that I would get a resulting url like "mysite/fish?content=trout"
I can't seem to figure out what the correct way to do this would be
though, and I don't see anything like this in any of the zpt docs.

Adam