# Proving partial correctness of the SetGCD algorithm in the hyperbook

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### Jens Weber

Jun 23, 2016, 8:55:39 PM6/23/16
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Hello TLA community,

I am trying to prove partial correctness of the SetGCD algorithm in the hyperbook - but I am not successful. Does anybody have a solution here? I am currently stuck on showing that S' is a finite set.

Thanks
Jens

### Stephan Merz

Jun 24, 2016, 2:15:34 PM6/24/16
I haven't tried writing a formal proof of that algorithm, and it is not entirely clear to me where you are stuck.

The (type) invariant contains the conjunct

IsFiniteSet(S)

and you need to prove that this predicate is preserved by action Lbl_1. You'll need to use the standard module FiniteSetTheorems contained in the TLAPS distribution. (If you haven't done so yet, you should add the corresponding directory to the Toolbox search path.) In particular, the lemmas FS_AddElement and FS_RemoveElement will be useful.

Hope this helps,
Stephan

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### Jens Weber

Jun 24, 2016, 2:30:09 PM6/24/16
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Thanks Stephan.
Yes, I also thought that I needed some axioms regarding removing and adding elements to finite sets. I didn't know about the FiniteSetsTheorem module though. I tried to define my own. Below is my current theorem. I'll look into the standard module now (but would still be interested in understanding why my axioms don't work.)

THEOREM ax1 == ASSUME NEW x \in Int PROVE IsFiniteSet({x})

THEOREM ax2 == ASSUME NEW x \in Int, NEW y \in Int PROVE IsFiniteSet({y-x})

THEOREM ax3 == ASSUME NEW S1, NEW S2,

/\ IsFiniteSet(S1)

/\ IsFiniteSet(S2)

PROVE /\ IsFiniteSet(S1 \ S2)

/\ IsFiniteSet(S1 \cup S2)

THEOREM Spec => []PartialCorrectness

<1>1. Init => SInv

BY InputOK DEF Init, SInv, TypeOK, PartialCorrectness

<1>2. SInv /\ [Next]_vars => SInv'

<2> SUFFICES ASSUME SInv /\ [Next]_vars

PROVE  SInv'

OBVIOUS

<2>1. TypeOK'

<3>1. CASE Lbl_1

<4>1. (S \subseteq Nat \{0})'

BY <3>1 DEF SInv, TypeOK, Next, vars, Lbl_1

<4>2. (S # {})'

BY <3>1 DEF SetGCD, SInv, TypeOK, Next, vars, Lbl_1, SetMax, Divides

<4>3. IsFiniteSet(S)'

BY <3>1, ax1, ax2, ax3 DEF SInv, TypeOK, Next, vars, Lbl_1

<4>4. QED

BY <4>1, <4>2, <4>3 DEF TypeOK

<3>2. CASE pc = "Done" /\ UNCHANGED vars

BY <3>2 DEF PartialCorrectness, SInv, TypeOK, vars

<3>3. CASE UNCHANGED vars

BY <3>3 DEF Init, SetGCD, PartialCorrectness, SInv, TypeOK, Next, vars

<3>4. QED

BY <3>1, <3>2, <3>3 DEF Next

<2>2. (SetGCD(S) = SetGCD(T))'

BY InputOK DEF Init, SInv, TypeOK, Next, vars

<2>3. PartialCorrectness'

BY InputOK DEF Init, SInv, TypeOK, Next, vars

<2>4. QED

BY <2>1, <2>2, <2>3 DEF SInv

<1>3. SInv => PartialCorrectness

BY DEF SInv, PartialCorrectness

<1>4. QED

BY <1>1, <1>2, <1>3, PTL DEF Spec

### Stephan Merz

Jun 24, 2016, 2:45:38 PM6/24/16
Hard to tell without being able to replay the proof. (If you send me your TLA module by private email I can have a look.)

Note that ax2 is subsumed by ax1 since it asserts that some particular singleton set is finite.

Regards,
Stephan

### Jens Weber

Jun 24, 2016, 2:54:16 PM6/24/16

Yes,  I initially just added ax1 and ax3 (then called “ax2”) for my proof – but when that didn’t work (and I hadn’t any other idea as to why not) I added the additional axiom (ax2), despite feeling that it shouldn’t be needed.

(I am new to TLAPS – so I am still learning the ropes.)

I have attached the module.

Thanks

Jens

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SetEuclid.tla

### Stephan Merz

Jun 25, 2016, 5:46:19 AM6/25/16
Hello Jens,

I changed two things: first, generalized your ax1 to

THEOREM ax1 == ASSUME NEW x (*\in Int*) PROVE IsFiniteSet({x})

so that one doesn't have to prove that y and y-x are integers (this follows of course from the type invariant but probably you'd have to make this explicit in the proof). Second, helped the prover by inserting a step (the first level-5 step below) that asserts that the expression on the right-hand side of the assignment is again a finite set. With these changes, the proof of TypeOK works as follows:

<2>1. TypeOK'
<3>1. CASE Lbl_1
<4>1. (S \subseteq Nat \{0})'
BY <3>1 DEF SInv, TypeOK, Next, vars, Lbl_1
<4>2. (S # {})'
BY <3>1 DEF SetGCD, SInv, TypeOK, Next, vars, Lbl_1, SetMax, Divides
<4>3. IsFiniteSet(S)'
<5>. \A x \in S : \A y \in {s \in S : s > x} : IsFiniteSet((S \ {y}) \cup {y-x})
BY ax1, ax3 DEF SInv, TypeOK
<5>. QED  BY <3>1 DEF SInv, TypeOK, Lbl_1
<4>4. QED
BY <4>1, <4>2, <4>3 DEF TypeOK
(* -- subsumed by <3>3
<3>2. CASE pc = "Done" /\ UNCHANGED vars
BY <3>2 DEF PartialCorrectness, SInv, TypeOK, vars
*)
<3>3. CASE UNCHANGED vars
BY <3>3 DEF Init, SetGCD, PartialCorrectness, SInv, TypeOK, Next, vars
<3>4. QED
BY <3>1, (*<3>2,*) <3>3 DEF Next

I agree that one would expect these modifications to be unnecessary. Unfortunately, at this point, the backends need a little more help than we'd like.

Best regards,
Stephan

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