Allow OPERATION clause in GROUP BY

Problem to solve

Allow OPERATION clause in GROUP BY

Further details

Simple example:

<group-by  field="MngAdsSty + ImdSty"/>

Complex example

<group-by  field="CASE CAST(s.MngAdsSty AS INT) WHEN 0 THEN -1 ELSE CAST(s.MngAdsSty AS INT) END + CAST(s.ImdSty AS INT)"/>

Proposal

The solution would be to be able to allow OPERATION clause like this

<group-by>
  <operation operator="ADD" >
    <field id="MngAdsSty" />
    <field id="ImdSty"/>
  </operation>
</group-by>