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*From*: Sidharth Kshatriya <sid.ks...@xxxxxxxxx>*Date*: Mon, 4 Aug 2014 04:25:50 -0700 (PDT)*References*: <85eb0aeb-0599-4dbe-8e3a-ac60d793ba94@googlegroups.com> <2d936027-2c40-4152-b58b-f94ac257148d@googlegroups.com>

Thanks for your quick explanation. "any one of them may be executed" --> does that mean that all of them that are enabled are executed? Lets take: Action1 == (x' = x + 5) Action2 == (y' = y + 5) What does Next == Action1 \/ Action2 here mean? (Note that I've deliberately omitted any preconditions for both Actions so that they are always enabled) Does it mean that *both* x and y increase in value by 5 or does it mean that only one of them increases in value by 5 "simultaneously" in a single time step? On Monday, 4 August 2014 16:46:52 UTC+5:30, Stephan Merz wrote: > Dear Sidharth, > > > you shouldn't really think of a TLA specification as being executed: it is a description of the possible behaviors of your system. (Although it turns out that TLC can effectively compute the behaviors for the subset of TLA+ that it accepts.) Then a disjunction A \/ B \/ C in the next-state relation means that the transition from the current state to the next one should satisfy (at least) one of the action formulas A, B or C. If several actions are enabled, either can one can actually happen, so "any one of them may be executed". And yes, > > > \E i \in 1 .. 4 : A(i) > > > is the same as > > > A(1) \/ A(2) \/ A(3) \/ A(4). > > > For example, think of A(i) being an action that models the input of value i to the system, then it is very reasonable to expect that the system should accept any value (in the domain of the inputs). > > > During verification, TLC will compute all possible branches at such a point and explore the subsequent behaviors. (That's where the state explosion problem in model checking comes from.) When evaluating the next-state relation, TLC computes the sets of all successor states (i.e., valuations of the primed variables) that make the formula true. Therefore it will indeed evaluate all the operands of a disjunction – note that this problem is different from a simple evaluation of a propositional formula to a Boolean value, since the values of the primed variables are initially unknown. > > > Hope this helps, > > > Stephan > > > On Monday, August 4, 2014 12:27:39 PM UTC+2, Sidharth Kshatriya wrote:Hi Folks, > I have a question regarding \/ . What does \/ mean when a system is actually executing a specification? > Say, > Next == Action1 \/ Action2 \/ Action3 > Now, lets say, Action1 and Action2 might both be enabled at a particular point of execution of an algorithm (i.e. they are non interleaving). Note that each action may relate primed and unprimed variables to each other. > Now does Next step here mean that Action1 *and* Action2 will be executed? Or does it mean that the *both* may be executed or any one of them many executed? > The reason I am confused is because: > TRUE == TRUE \/ DONTCARE > TRUE == TRUE \/ FALSE > TRUE == TRUE \/ TRUE > Basically is short circuit evaluation implicit in the execution semantics? Do we evaluate all operands of an \/? > Similarly we may have: > Next == \E i \in 1..4 Action(i) > Is this the same as: > Next == Action(1) \/ Action(2) \/ Action(3) \/ Action(4) > > Thanks, > Sidharth

**Follow-Ups**:**Re: Execution semantics of \/***From:*Sidharth Kshatriya

**References**:**Execution semantics of \/***From:*Sidharth Kshatriya

**Re: Execution semantics of \/***From:*Stephan Merz

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