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[tlaplus] Re: Usage of the ! operator

The third meaning of ! is for naming subexpressions of an _expression_ and is intended only for use in 
proofs.  It is explained in section 6 of
TLA+ Version 2 A Preliminary Guide


On Monday, August 19, 2019 at 12:24:37 PM UTC-7, Saswata Paul wrote:

  According to the book, I could find only two usages of the ! operator :- in EXCEPT and in operator names.

 However, in the formal proof of paxos (https://github.com/tlaplus/v1-tlapm/blob/master/examples/paxos/Paxos.tla), on line 250, the ! operator has been used and I am not quite clear about the context in this case.

 I will greatly appreciate it if someone can explain the meaning of this particular line of code: 

<2>1. PICK m \in msgs : Phase2b(a)!(m)   


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