[tlaplus] The identifier is undefined or not an operator

["'Amine BOUFTILZ' via tlaplus" <tlaplus@xxxxxxxxxxxxxxxx>]

Hello everyone,

I'm working on a project where Indiana Jones and his team need to cross a bridge. I wrote some code, but I ran into this error:  

The identifier left is undefined or not an operator: at line 24 in the method DeuxActeursTraversent. 

I even tried simplifying Next to something like left' = left, but it still doesn’t work.  

Any ideas?

---------------------- MODULE Indy ------------------------------------------
EXTENDS Naturals, TLC

tau == [ I |-> 1, G |-> 2, W |-> 4, K |-> 8, L |-> 0 ]
maxt == 15 
A == DOMAIN tau  
L == "L" 
N == Nat

max(S) == CHOOSE x \in S : \A y \in S: y \leq x
Max(S) == max( {tau[a] : a \in S} )

ASSUME
        /\ \A x \in A : tau[x] \in N
        /\ maxt \in N
        /\ L \in A

VARIABLES left, time

Init ==
  /\ left = A
  /\ time = 0

DeuxActeursTraversent ==
                            \E a, b \in left : (left' = left \ {a,b,L}) /\ (time' = time + Max({tau[a], tau[b]}))

UnActeurRevientAvecLampe ==
                            /\ \E a \in A \ left : (left' = left \union {a,L}) /\ (time' = time + tau[a])

Next == DeuxActeursTraversent \union UnActeurRevientAvecLampe

Inv ==
    /\ left \subseteq A
    /\ time <= maxt  
 
Target ==
  left = {} 
  /\ time <= maxt 

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

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