How to control order of emphasis indicators for multiple emphasis?

[bertfrees]

Currently the order of the indicators is determined by the order of the emphasis class definitions in the table. But what if the caller wants to have control?

[bertfrees]

Actually we also make sure that the order of the closing indicators is always the inverse of the order of the opening indicators. So if e.g. italic begins before bold, and they end at the same place, the order of the closing indicators is determined by the order of the opening indicators. Something similar happens when e.g. italic ends after bold but they begin at the same place.

The only case where the user is not in control is when no type of the emphasis begins or ends before or after the other types of emphasis.

[bertfrees]

This is not a trivial problem to solve and I'm not convinced of the added value, because I can't really see the difference in semantics between <b><i>foo</i></b> and <i><b>foo</b></i>.