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[tlaplus] Beginner question: EXCEPT construct

Suppose the next state relation for the function "state" is:

state' = [state EXCEPT ![A] = "newState"]

where A is some argument of the function "state". This obviously works when A is a single argument.

What about when A is a "set of arguments" (i.e., A = {"A1", "A2", "A3"})? Then the expression:

state' =  [state EXCEPT ![A] = "newState"]     

is not correct: TLC gives an error message: 

"TLC threw an unexpected exception.
This was probably caused by an error in the spec or model. See the User Output or TLC Console for clues to what happened. The exception was a java.lang.RuntimeException: Attempted to check equality of string "A1" with non-string: {"A1"}

The error occurred when TLC was evaluating the nested
expressions at the following positions:"

Clearly, I am missing something obvious. How could it be possible to resolve this?

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