We propose a generalisation of Winskel's event structures, matching the expressive power of arbitrary Petri nets. In particular, our event structures capture resolvable conflict, besides disjunctive and conjunctive causality. With every event structure we associate a family of configurations, a propositional theory and a Petri net. We characterise several classes of event structures in terms of the axiomatisability of the associated propositional theories by formulae of simple prescribed forms, and in terms of structural properties of the associated Petri nets.

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