You don't need to write any code. All you need to do is write BRF+ rules. You can execute the derivation node under the "Search and Validation" tree, enter the data model name, and create a BRF+ function. The derivation function must be created under the Derivation folder and must be named DERIVE_<entity>. The check function must be created under the corresponding folder and must be named CHECK_<entity>. So, for the MARABASIC, you must call the derivation function DERIVE_MARABASIC and the check method CHECK_MARABASIC.
Keep in mind, BRF+ derivation and validation functions can handle one entity at a time. For cross-entity validation and derivation, you need to implement the cross-entity BAdI (in which you will need to write code).
Take a look at these documenets: http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/e08f3e4e-17f8-2f10-f383-fe126bffe241?overridelayout=t… & http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/90243ca8-92af-2e10-b895-92533b2de0ce?QuickLink=index&…