[Agda] syntax declaration

[Michel Levy]

I made the following statement

data exists (A : Set)(P : A -> Set): Set where
  <_,_> : (a : A) -> P a -> exists A P

I would like to rewrite (exists A P) as, for example,  (ex (x : A) P x).
But I don't understand how to do it. I tried :

syntax exists A P   = ex  (x : A) , P x

but I had the mistake : syntax must use holes exactly once

Can you explain to me how to use the syntax declaration on this example ? What are these holes in my syntax statement ?

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