# [tlaplus] Simple exercise: hanoi

Hi,

This is my first complete and working specification. I'll be grateful for your comments.

------------------------------- MODULE hanoi -------------------------------

EXTENDS Integers, Sequences

VARIABLES a, b, c

Empty(seq) == Len(seq) <= 0

Cons(elem, seq) == << elem >> \o seq

AB == /\ ~Empty(a)
/\ Empty(b) \/ Compare(a, b)
/\ a' = Tail(a) /\ b' = Cons(Head(a), b) /\ c' = c

AC == /\ ~Empty(a)
/\ Empty(c) \/ Compare(a, c)
/\ a' = Tail(a) /\ b' = b /\ c' = Cons(Head(a), c)

BA == /\ ~Empty(b)
/\ Empty(a) \/ Compare(b, a)
/\ a' = Cons(Head(b), a) /\ b' = Tail(b) /\ c' = c

BC == /\ ~Empty(b)
/\ Empty(c) \/ Compare(b, c)
/\ a' = a /\ b' = Tail(b) /\ c' = Cons(Head(b), c)

CA == /\ ~Empty(c)
/\ Empty(a) \/ Compare(c, a)
/\ a' = Cons(Head(c), a) /\ b' = b /\ c' = Tail(c)

CB == /\ ~Empty(c)
/\ Empty(b) \/ Compare(c, b)
/\ a' = a /\ b' = Cons(Head(c), b) /\ c' = Tail(c)

Init == /\ a = << 1, 2, 3, 4 >>
/\ b = <<>>
/\ c = <<>>

Next == AB \/ AC \/ BA \/ BC \/ CA \/ CB

Spec == Init /\ [][Next]_<<a, b, c>>

=============================================================================

Please note: the goal was to make it as simple as possible, I've done it together with my son, to show him how math and computers can solve real world problems.

Best regards,
Emanuel

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