  Event structures are fundamental models in concurrency theory, providing a
representation of events in computation and of their relations, notably
concurrency, conflict and causality. In this paper we present a theory of
minimisation for event structures. Working in a class of event structures that
generalises many stable event structure models in the literature (e.g., prime,
asymmetric, flow and bundle event structures), we study a notion of
behaviour-preserving quotient, referred to as a folding, taking (hereditary)
history preserving bisimilarity as a reference behavioural equivalence. We show
that for any event structure a folding producing a uniquely determined minimal
quotient always exists. We observe that each event structure can be seen as the
folding of a prime event structure, and that all foldings between general event
structures arise from foldings of (suitably defined) corresponding prime event
structures. This gives a special relevance to foldings in the class of prime
event structures, which are studied in detail. We identify folding conditions
for prime and asymmetric event structures, and show that also prime event
structures always admit a unique minimal quotient (while this is not the case
for various other event structure models).
