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Re: [tlaplus] Very simple question by TLA newcomer

Dear Petar,

welcome to this group.

In order to answer your question: the SMT backend of the proof system is not able to prove temporal logic formulas. You should decompose your proof as follows:

THEOREM Spec => []Invariant
<1>1. Init => Invariant
<1>2. Invariant /\ [Next]_<<CarsGo, PedestriansGo>> => Invariant'
<1>. QED
   BY <1>1, <1>2, PTL DEF Spec

The non-temporal steps <1>1 and <1>2 should be provable using SMT, but the PTL backend has to be used for the final step. You may also want to read the tutorial on the TLAPS Web site.


However, I am not sure if it is a good idea to introduce the proof system for your presentation. I would imagine that you are better off showing the model checker, all the more for a finite-state specification such as yours.

Best regards,

On 23 Apr 2016, at 19:43, Petar Vukmirovic <petar.vu...@xxxxxxxxx> wrote:

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.

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