About Enabled semantics

[Huailin]

Team,

I got a quick question about the semantics of Enabled of TLA+. Thanks for the clarification in advance.

As we know, the Enabled Predication is defined as:

s[Enabled A] = Exist t \Exist St: s[[A]]t. 

For the action A, there exists a state t in the state space St, so that action A could be possible to get a chance(may not get the chance, though) to execute.

Then, my question is:  When/after Enabled(A) is TRUE, when it could become false, or we say, why/when the state "t" will run away....? so that we can not match the s[[A]]t any more.

Can you give an example? t run away for a vacation, and then, after a while, come back...then Enabled(A) becomes TRUE again.

Thanks,

Michael

[Hillel Wayne]

Consider

A == x /\ x' = FALSE
B == ~x /\ x' = TRUE

Then ENABLED A <=> ~(ENABLED B).

H

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