Help to implement accept and reject functionalities of PO Approval .

HI all,

I am trying to redefine the inherited methods of  ZCL_Z_PORDER_02_DPC_EXT  Data Provider Class. And i have redefined the

GetEntityset methods.

PURCHASEORDERHEA_GET_ENTITYSET of   PurchaseOrderHeaderCollection EntitySet and 

  PURCHASEORDERITE_GET_ENTITYSET of  PurchaseOrderItemCollection   EntitySet .

Following are the URL's to access  purchase order  and purchase item information.

http://<hostname>:<port>/sap/opu/odata/sap/Z_PORDER_SRV_02/PurchaseOrderHeaderCollection

http://<hostname>:<port>/sap/opu/odata/sap/Z_PORDER_SRV_02/PurchaseOrderItemCollection?$filter=(PONumber eq '4500000003')

But i am finding difficulty to implement  accept and reject functionalities of PO Approval . Please help me to

redefine the methods to achieve the accept and reject functionalities of PO Approval .