THIS FILE IS OBSOLETE - SHOULD BE MERGED WITH TODO

group := { (non-continugous) fields }

1) number items
2) collect items into groups (including replication, attachment of different
   values)
3) modifiable selector: list of valid values and (group) selections,
   plus negative-groups (optional)
4) field may be in multiple, mutually exclusive groups
   -> collision check:
     find collider(s), try to trace to common exclusive root


Restrictions
------------

 - there's only very limited error checking
 - no support for arrays (repeated IEs)
 - no support for variable-length items
 - copy operations are not optimized
 - default case must be empty
