[rules-users] Design of Rule Engine

Hi, I am using the 5.x version of Drools and currently new. I am currently evaluating Drools for an internal project . The product's impressive and the support is absolutely superb. To design a new Rule Engine using Drools as the base are there any suggested design patters that i can take a look at ? If there are any best practices that should be followed and gotchas to be avoided  it would be great if i could read about them. For example if i am writing rules where two files contain a common set of rules, is it possible to abstract out the common set of rules to a seperate DRL and have that DRL referenced in other DRL's ? These and other similar questions are what i am seeking answers to . Regards, Manav