Firing on Fleeing Units

[cjsiam]

It surprised us that there is nothing that says "Thou shalt not"....

So---Can you?  Can you exercise Opportunity Fire on a unit which failed his Demoralized Recovery
  and is forced to flee out of range?

Shooting and killing tanks, one wants as many shots as you can....and so firing at a tank as it has to "run away" is
  a shot one would like to take....

But----I'm not sure we can or cannot.

We could not find anything that said you could or could not....just ...didn't come up in any relevant sections...

Is there something obvious ???
(Germans mowing down my hapless russians....I mean, it's in character and everything....)
cj