Diference in "Derivative Security List" message's definition in FIX and FIXML

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Imported from previous forum

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[ original email was from Olexiy Getmanchuk - alexeyg@in4reach.com ]
Hello All!

We have noticed one strange difference in definition of "Derivative Security List"(AA) message in FIX and FIXML specifications.
In FIX we have repeating group:
…
146 NoRelatedSym
-> "Component block <Instrument>"
-> 15 Currency
-> 555 NoLegs
-> -> Component block <InstrumentLeg>
-> -> 556 LegCurrency
…

As you see NoLegs repeating group is nested inside NoRelatedSym rep. group.

BUT in FIXML we have:

<!ENTITY % DerivSecurityListContent … InstrumentList?, InstrumentLegList?..">

Which means that InstrumentList (NoRelatedSym in FIX) and InstrumentLegList (NoLegs in FIX) are at the same level inside the message and are not nested.

How it could be?