Double nested fieldsets with Dexterity?

We have a fairly complex content-types with about 100 fields where the fields are already grouped into fieldsets.
However we need another level of fieldsets inside a fieldset. E.g. the fieldset "contact" should provide fields for private and business address and therefore the fieldset "contact" should contain two sub fieldsets "private address" and "business address" with their related fields. The challenge on one head is to express the nested fieldsets either on the supermodel XML level or within the Python-based schema definition and on the other hand to render such a content-type (edit and view mode).

Any ideas?


from it hints that z3c.form Groups functionality is still supported but isn't as obvious to use

Or maybe subforms would work too:

Also this (but I haven't looked)