[rules-users] Monitoring sensor parameter change

[Bartek]

Hello,
I am using Drools with Guvnor and Drools Fusion.  I have application 
that read data from sensors and after each read insert event to 
StatefulKnowledgeSession. I have some basic rules that works.
Now I need to detect when one of sensor parameter change by some value 
between reads from given sensor, e. g.

Lets say I want to detect when power rise by more then 2:
4 sensor reads (4 events inserted to session)
1. power=0.9
2. power=1
3. power =4 --> rule should detect that fact (4-1=3 ; 3>2)
4. power = 2

I do not now how to get access to previous reads in rule to do 
subtraction (4-1=3).  Any advice appreciated.

Thanks,
Bartek