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*From*: Leslie Lamport <tlaplus.ll@xxxxxxxxx>*Date*: Sun, 10 Mar 2019 12:52:43 -0700 (PDT)*References*: <6e375878-19a2-4ea0-b792-a056e921ce86@googlegroups.com>

(1) If we remove the Inv from the formula Inv /\ Next => ..., it would assert

that a step starting in any state that satisfies Next satisfies "..." --

for example a state in which memQ is a sequence of imaginary numbers.

I have no idea if that assertion is true for such a starting state.

However, it suffices to prove the assertion for steps starting in a

reachable state. Conjoining the invariant Inv allows you to prove

the assertion only for reachable states. You have to choose Inv so

it asserts what is true about reachable states that makes the

implication true. To do this, you have to understand why the theorem

you're trying to prove is true.

that a step starting in any state that satisfies Next satisfies "..." --

for example a state in which memQ is a sequence of imaginary numbers.

I have no idea if that assertion is true for such a starting state.

However, it suffices to prove the assertion for steps starting in a

reachable state. Conjoining the invariant Inv allows you to prove

the assertion only for reachable states. You have to choose Inv so

it asserts what is true about reachable states that makes the

implication true. To do this, you have to understand why the theorem

you're trying to prove is true.

(2) That mapping isn't derived; you have to invent it. The sentence

beginning "Intuitively" that starts on line 9 of page 63 tells you

what condition that substitution must satisfy. To be able to choose

the necessary mapping, you need to understand why the theorem you're

trying to prove is true .

beginning "Intuitively" that starts on line 9 of page 63 tells you

what condition that substitution must satisfy. To be able to choose

the necessary mapping, you need to understand why the theorem you're

trying to prove is true .

Leslie

On Wednesday, March 6, 2019 at 10:53:35 PM UTC-8, Oliver Yang wrote:

On Wednesday, March 6, 2019 at 10:53:35 PM UTC-8, Oliver Yang wrote:

Hi All,In Section 5.8 of book "Specifying Systems", the "Proving Impl" is introduced. I have a rough understanding of refinement mapping, whichessentially maps states of Spec A to the states of Spec B. However, I have a hard time understanding "step simulation".1) What's the purpose of introducing the invariant Inv in Formula 5.3? What are we trying to achieve here?2) How do we derive the mapping: omem = vmem, octl = ..., obuf = buf? It looks like we jumped to the conclusion without showing any proof?Thanks,Oliver

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**Follow-Ups**:**[tlaplus] Re: How to understand the concept "step simulation"***From:*ss . nedunuri

**[tlaplus] Re: How to understand the concept "step simulation"***From:*Oliver Yang

**References**:**[tlaplus] How to understand the concept "step simulation"***From:*Oliver Yang

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