begin-of-transaction in ABAP Objects

Hi,

I have a question regarding the (implicit or explicit) begin of a transaction (read: SAP LUW) in ABAP Objects. In this piece of code

 ...
CREATE OBJECT mapping.
TRY.
  mapping->delete_all_embraced_mappings( ).
  mapping->merge_with_begda_overlap( ).
  mapping->merge_with_endda_overlap( ).
  mapping->store( ).
  COMMIT WORK.
CATCH /my/mapping_exception.
  ROLLBACK WORK.
ENDTRY.  

I need a all-or-nohting logic, i.e. I'd like all the methods <i>delete..(), merge..()</i> and <i>store()</i> to run in a single transaction. If any of the methods throws a <i>/my/mapping_exception</i> I want all DB changes (carried out in the methods) rolled back.

Now, how can a mark the <b>begin-of-transaction</b> (the counterpart of the commit work)? I couldn't find information about this in the docu or in the SDN. To prevent misunderstandings: I am building a web application with ABAP Objects, so no dialog steps, PBO-PAIs, function modules, performs etc. are involved!

Regards,

Sebastian Kamp