define 'change' event for BUS2054 - wbs element

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Dear workflow people
 
we want to define a new event for business object BUS2054 (wbs element =
in the PS)
 
the only event defined in standard is the 'created' event
 
can someone give me guidance on how to define a 'changed' event ?
 
the purpose is to use this as a triggering event for WF, each time a WBS =
element is changed
 
many thanks in advance
 
ronen