Intermediate commit to prove that UDPOR functions

Intermediate commit to prove that UDPOR functions

This commit returns a const_cast<>-ed UnfoldingEvent
from the intersection of the sets A and ex(C) in
the Unfolding. We use this work around currently as
there is no way currently to distinguish between a
set of events and a set of constant events with
EventSet. To do so would require a number of changes
and probably a class that would be entirely duplicated
and more difficult to read without the help of e.g.
a template (which itself is not always the easiest to
read).