from drools documentation:

Sequential mode can only be used with a StatefulSession and is off by default. To turn on either set the RuleBaseConfiguration.setSequentail to true or set the rulebase.conf property drools.sequential to true.

when you use it:

java.lang.RuntimeException

: Cannot have a stateful rule session, with sequential configuration set to true

somebody there who can solve this mytery?

Cheers

Markus