# [tlaplus] [External] Need help to understand why model checker is ending up in stuttering step

Hi Folks,

I am new to TLA+ and trying to write a spec for checking cycle in a singly linked list.
The model checker is ending up in stuttering. Can anybody please help me understand & fix it? Thanks!

Here is the spec: -

--------------------------- MODULE CycleDetection ---------------------------

EXTENDS TLC, Naturals, FiniteSets, Sequences

CONSTANT N, NIL

\* Put the constraint on N

ASSUME NIsPositive == N \in (Nat \ {0})

\* Put the constraint on NIL

ASSUME NilIsNotInN == NIL \notin 1..N

\* Represents the set of nodes

Nodes == 1..N

\*Init position of hare and tortoise

InitPos == 1

VARIABLES linkedList, tortoise, hare, cycleDetected, pc

vars == << linkedList, tortoise, hare, cycleDetected, pc >>

IsValidLinkedList(l) == \A n \in Nodes :

\/  /\ n = N

/\ l[n] \in ((Nodes \union {NIL}) \ {n})

\/ l[n] = n + 1

ValidlinkedListsSet ==  {l \in [Nodes -> Nodes \union {NIL}] : IsValidLinkedList(l) }

/\ tortoise = InitPos

/\ hare = InitPos

/\ cycleDetected = FALSE

/\ pc = "MoveTortoise"

MoveTortoise ==

/\ pc = "MoveTortoise"

/\ tortoise' = IF tortoise # NIL THEN linkedList[tortoise] ELSE NIL

/\ pc' = "MoveHare"

/\ UNCHANGED << linkedList, hare, cycleDetected >>

MoveHare ==

/\ pc = "MoveHare"

/\ pc' = "Check"

/\ UNCHANGED << linkedList, tortoise, cycleDetected >>

Check ==

/\ pc = "Check"

/\ cycleDetected' = (tortoise = hare)

/\ pc' = IF (tortoise = NIL \/ hare = NIL \/ (tortoise = hare)) THEN "Done" ELSE "MoveTortoise"

/\ UNCHANGED << linkedList, tortoise, hare >>

Next == MoveTortoise \/ MoveHare \/ Check \/ (pc = "Done" /\ UNCHANGED vars)

Spec == Init /\ [][Next]_vars

TypeInvariant ==

/\ pc \in {"MoveTortoise", "MoveHare", "Done", "Check"}

/\ cycleDetected \in BOOLEAN

/\ tortoise \in (Nodes \union {NIL})

/\ hare \in (Nodes \union {NIL})

Termination == <> (pc = "Done")

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

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