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# Re: Very simple question by TLA newcomer

Dear Mr Lamport and Mr Merz,

I am really sorry I did not respond earlier, but somehow I did not get any notifications for this post.

Anyway, thank you very much for your help and answers, I will take a look at the presentation you proposed.

The presentation is half an hour presentation on a special topic in software testing (such as TLA+)
aimed towards first year Master CS students.

Kindest regards,
Petar Vukmirovic.

субота, 23. април 2016. 21.07.21 UTC+2, Petar Vukmirovic је написао/ла:
Dear all TLA lovers,

I am very new to TLA, but I read the Amazon article and I was very much intrigued by this tool so I wanted to learn more.

In one of my masters courses we learning about modelling specifications and trying to generate test cases from them or proof some of our assumptions about specifications.

I talked to my professor about TLA+ and she liked it as well, so she told me to prepare a short introduction to it on Special Topics class. My task is to create a simple model of a sempaphore in TLA+.

To this end, I created the following TLA+ specification:

----------------------------- MODULE semaphore -----------------------------
VARIABLES CarsGo, PedestriansGo

\* Initial state
Init == \/ /\ CarsGo = TRUE
/\ PedestriansGo = FALSE
\/ /\ CarsGo = FALSE
/\ PedestriansGo = TRUE

\* Next step
Next == /\ CarsGo' = ~CarsGo
/\ PedestriansGo' = ~PedestriansGo

Spec == Init /\ [][Next]_<<CarsGo, PedestriansGo>>

Invariant == CarsGo # PedestriansGo

----------------------------------------------------------------------------
THEOREM Spec => []Invariant
BY DEF Spec, Invariant
============================================================================

However, when I try to prove the theorem, I get the message:
(* SMT failed with status = sat
*).

Can someone explain me what I am doing wrong?

Kindest regards,
Thanks in advance,

Petar Vukmirovic.