PropCheck.TargetedPBT.exists

You're seeing just the macro exists, go back to PropCheck.TargetedPBT module for more information.
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exists(arg, list)

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The exists macro uses the targeted PBT component of PropEr to try to find one instance of xs that makes the prop return true.

If such a xs is found, the property passes. Note that there is no counterexample if no such xs could be found.