[rules-users] newbie question: forming event pairs

Hi all, I'm new to drools and I have the following problem: A stream of Start and Stop events comes into the engine. The start and stop events should be paired and transformed into ranged events. Each start event should be paired to the first next stop event that is not part of a pair already, with precedence for the last start event that came in. The problem is comparable to what a parser would do to match nested braces. A sequence of the form 1: Start 2: Start 3: Start 4: End 5: Start 6: End 7: End 8: End should form the following pairs 3-4 5-6 2-7 1-8 I checked the documentation but I still have two problems: 1. how can I find the next stop event (or previous start) 2. how can I make sure the pairs are formed innermost first? Thanks in advance, Wouter