[rules-users] How to write a rule like this

Hello, I'm still quite new to the drools stuffs just getting the examples running. I'm sorry if my question is too basic. I have a system that will continuously to receive events sent from some other system. Basically I have two types of events, eventA and eventB. Both eventA and eventB have two properties, index and timestamp. What I want is to capture a pattern to meet the following conditions 1. every 3 consecutive eventsA without any eventB in between having a same index value ordering by their timestamp. 2. all events in a matched pattern will have the same value of property index. 3. if a pattern is matched, any of the events in this pattern should not be used again for a new matching. there are some examples below, the number after a colon stands for the value of its index number and assume those events are already ordered by their timestamp. eventB:2 eventA:1 eventA:1 eventA:1 eventB:3 --------- one successful match, we have three eventA with same index value 1 in a row eventB:2 eventA:1 eventA:1 eventB:1 eventA:1 --------- no match because there's an eventB in between eventB:2 eventA:1 eventA:1 eventB:2 eventA:1 --------- one successful match because the in-between eventB has a different index value eventA:1 eventA:1 eventA:1 eventA:1 eventA:1 --------- only one match, because once matched, the events can not be used for a second match again. I just don't know how to write a rule like this. Is there anyone who can shed some lights on? Best Regards, Kevin