setup trinity bars like bartender?

Zanthor explained how to do this with bartender:


"Bartender Setup
The bartender setup is detailed here but the brief overview is as follows.
Open bartender's configuration by typing /bt3 config.
Expand the bar you want to setup for spells and click on "button paging"
Set this to "Advanced"
Click "Edit States"
Add a new state
For condition put target=focustarget,exists
For unit put focustarget
You can do this for 1 bar, or 10 bars, I normally just have one that is handled this way, I dont waste "targetless" spells on it, such as totems, etc. The way this works is that if you have a focus who has a target you'll automatically assist them, if not, you'll just cast the spell as normal. Makes it useful to play solo or grouped."

Anyone know how to do this with trinity bars?