topic-info-types too restrictive ?

Working on a DITA implementation in FrameMaker I stumble over the restriction that a topic only allows topic elements to be nested. I would have expected the topic to allow specialisations of topic (such as concept, task and reference and possibly troubleshooting) as well in that location. Of course every type of nesting can (and probably should) be done with maps instead of nesting topics, but the standard does allow topic nesting. So if people want to use this, they should be able to use specialised topics as well, without having to redefine the topic-info-types. Would such a redefinition be legal (assuming I also add the modules for concept, task, reference and troubleshooting to the topic document shell so that all elements are defined) ? Would that count as a specialised topic ? Are there points I am missing in the original discussions of nesting topic types ?

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