This is a question regarding infinite loops and shortcuts at **Competitive REL**

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here is a potential scenario.

- Both my opponent and Myself are at 20 life each.
- I have no creatures on the battlefiled.  
- My opponent has a [mtg: kiki-jiki, mirror breaker] and a [mtg: warstorm surge] on the battlefield
- I have a [mtg: rakdos charm] in hand.

- my opponent plays [mtg: deceiver exarch] and begins a loop where he uses Kiki-Jiki to copy the ex-arch, and he uses the new exarch to untap Kiki-Jiki, dealing me 1 damage every time he does it.  


- If I cast [mtg: rakdos charm] when the 18th exarch token's trigger is on the stack, I win.
- If I cast it at any other time, I lose
- If my opponent knows that I'm going to cast it than he won't make the 18th token, and he'll simply attack and kill me.

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Now the question is:  

If my opponent proposes a shortcut, saying

> "I'll make 500 tokens and kill you"

Can I accept the first 18 iterations of that shortcut and react by saying 

> "after the 18th token enters the battlefield and it's ETB trigger goes onto the stack, I cast [mtg:rakdos charm].

Or do I have to propose a new shortcut for the 18 iterations, potentially giving my opponent information?