dala-relation and protected

so, people ask me why I don't like datasets .. well .. could someone explain this:

given this class:

using Progress.Lang.*.

class datasetA:
def protected temp-table ActvtyData no-undo
namespace-uri "ActivityData"
field schemaVersion as decimal
xml-node-type "ATTRIBUTE" .

def protected temp-table MsgHdr no-undo
namespace-uri ""
field MsgTp as character
field FlNm as character
field ActvtyData_id as recid
xml-node-type "HIDDEN" .

define protected dataset ActvtyDataDset namespace-uri "ActivityData"
xml-node-type "HIDDEN"
for ActvtyData, MsgHdr
parent-id-relation relation1 for ActvtyData, MsgHdr
parent-id-field ActvtyData_id.

end class.

and this class:

using Progress.Lang.*.

class datasetB inherits datasetA:
end class.

A) classA compiles with no problem

B) classB fails to compile with the error RELATION-FIELDS phrase is required for DATA-RELATION definition (12386)

what exactly am I missing ? 

progress version is 11.3.1