# Re: [tlaplus] Understanding CHOOSE

Hello John,

I think Alex answered your first point, but I'll try to express by my own words for you to see how it translates to a formal description.
Say your system is a simple increment/decrement of a global state :

/\ i' = i + 1
/\ i' = i - 1

As you mentioned, you can use CHOOSE :

Init == i = 0
Next == LET answer == CHOOSE code \in {"200", "400"}: TRUE

But as mentioned by Alex and yourself, this would not yield the expected behavior. CHOOSE having no guarantees, all possible outputs are not covered by TLC.
The existential quantifier is the correct way of exploring all possible behavior :

Init == i = 0

The operator \E corresponds to the correct way of introducing non-determinist behavior in your specifications. TLC will check all possible behaviors for you.

I hope it helped,

Thomas
Le mercredi 20 janvier 2021 à 15:40:58 UTC+1, John a écrit :
Hi Alex,