Turns a pattern of patterns into a single pattern. (this is actually
join)
1/ For query
arc, get the events from the outer pattern
pp 2/ Query the inner pattern using the
part of the
outer 3/ For each inner event, set the whole and part to be the
intersection of the outer whole and part, respectively 4
Concatenate all the events together (discarding wholesparts that
didn't intersect)
TODO - what if a continuous pattern contains a discrete one, or
vice-versa?