How to increment variable in [[ ]] ?

Hello there, I'm fairly new to Twine, and I'm having a hell lot of fun.
I'd like to do something of that kind :

A damsell in distress !!

[[ Help -> Outcome]]
[[ Flee -> Outcome]]

And I'd like for the variable $courage to gain +1 if "Help" is chosen. How do I do ?
I've tried weird stuff like :

[[ Help -> Outcome ] (set: $courage to $courage + 1)]

But it didn't work. If there's anything I've explained badly, do say so !

Thanks in advance !