[rules-users] how to ensure that one rule only is executed once?

Hi I just try to uses jboss rules in our projects. I met a question. for example package sample rule "rule 1" when $o:Order (amount>100 and amount<200); then $o.setPrice($o.getPrice()-10); end rule "rule 2" when $o:Order (amount>200); then $o.setPrice($o.getPrice()-20); end if the fact , order's amount is 210. then the rule2 will be executed, then the rule1 will be fired . I want to stop the session once some rule is executed or how to control the rule flow. thanks in advance -Yong