Neutralizing Contingency: Ammianus Marcellinus as a Participant in Julian's Persian Campaign, 363 AD

What is the relation between history as contingent traumatic experience (Julian's Persian campaign), history as narrative of that experience (Res gestae of Ammianus Marcellinus) and the historian as author/narrator/participant (Ammianus Marcellinus)? The documentary dimension of the historian's experience, usually accepted at face value, is problematized. The historian's intermittent appearances in the text as participant are literary choices establishing the evidentiary value of the narrative.