The group structure is stored after the drawing objects. There are two
parts:
Part 1: Definition which drawing object belongs to which group
For each group which has at least one drawing object as a direct
member and not only other groups the following blank separated list
is given: The number of the group followed by the sequence numbers
of the drawing objects which are direct members. This list ends with
a zero. The sequence number of a drawing object is given by the list
of drawing objects.
Each drawing object can be a direct member of only one group. A
drawing object which is not member of a group is not listed.
Part 1 is concluded with a zero.
Part 2: Definition of the group hierarchy.
This part is a blank separated list of pairs of numbers concluded with
two zeros. Each pair gives a groupnumber and the number of the parent
group. Groups which do not have a parent group are not listed.
