  Event structures represent concurrent processes in terms of events and
dependencies between events modelling behavioural relations like causality and
conflict. Since the introduction of prime event structures, many variants of
event structures have been proposed with different behavioural relations and,
hence, with differences in their expressive power. One of the possible benefits
of using a more expressive event structure is that of having a more compact
representation for the same behaviour when considering the number of events
used in a prime event structure. Therefore, this article addresses the problem
of reducing the size of an event structure while preserving behaviour under a
well-known notion of equivalence, namely history preserving bisimulation. In
particular, we investigate this problem on two generalisations of the prime
event structures. The first one, known as asymmetric event structure, relies on
a asymmetric form of the conflict relation. The second one, known as flow event
structure, supports a form of disjunctive causality. More specifically, we
describe the conditions under which a set of events in an event structure can
be folded into a single event while preserving the original behaviour. The
successive application of this folding operation leads to a minimal size event
structure. However, the order on which the folding operation is applied may
lead to different minimal size event structures. The latter has a negative
implication on the potential use of a minimal size event structure as a
canonical representation for behaviour.
