VAT Ledger Czech 2016: Section A4 -> How to get invoice number instead of delivery note

Dear experts,

for VAT Ledger Czech 2016 and Section A4 invoice number is mandatory instead of delivery note (Default) SAP message: By default, the Czech VAT Ledger report expects the invoice number to be stored on the FI document in the field Reference (BKPF-XBLNR) for all the sections of the VAT Ledger return. If you do not want to export field Reference as the invoice number, you can implement method GET_INVNUMBER (Get Invoice Number) of BAdI IDFIVAT_APP_BADI and use your own algorithm do determine required invoice number from other data (BELNR, BKTXT, etc.) - please see SAP Note 2193486 - FAQ /regular update/ for Legal Change Is there anybody of you wo already did that kind of implementation and who is able to provide me the ABAP coding?

Thanks in advance.

BR, Bernd