Membrane system computations proceed in a synchronous fashion: at each step all the
applicable rules are actually applied. Hence each step depends on the previous one.
This coarse view can be refined by looking at the dependencies among rule occurrences, by recording,
for an object, which was the a rule that produced it and subsequently (in a later step),
which was the a rule that consumed it.
In this paper we propose a way to look also at the other main ingredient in membrane system
computations, namely the simultaneity in the rule applications.
This is achieved using zero-safe nets that allows to synchronize transitions,
i.e., rule occurrences.
Zero-safe nets can be unfolded into occurrence nets in a classical way, and to this unfolding an event structure can be associated. The capability of capturing simultaneity of zero-safe nets is transferred
on the level of event structure by adding a way to express which events occur simultaneously.|