Editing Graphics development with vplot (section)

==Groups==
The begin-end group commands are provided so that a "MacDraw"-like
vplot editor may be created. The device knows when groups are
opened and closed, and their positions within the current plot file,
via the dev.attributes(BEGIN_GROUP,...) and dev.attributes(END_GROUP,...)
calls. You can use fseek to reposition the plot stream to the beginning
of a desired group, and then call dovplot to plot the group again.
When the group open (number one, not zero) command is processed, the device
gets a chance to reset global attributes from their "initialized" values.
(For example, to change the color.) When the group close (number one)
command is processed, the device gets a chance to to reposition pltin
to the end of the file and so cause dovplot to think it's done and return.

It is possible for the user to violate the grouping laws, ie:
begin-group and end-group commands must be paired within a file,
erases and breaks may not be contained within a group.
Dovplot will warn the user if this happens. How the device handles
such an error, however, is up to it. Caveat user.

A group number 0 is generated by dovplot itself for each plot frame.
The group name gives the file name it came from with an appended number
counting multiple frames within one file. A frame begins whenever the
screen is erased or there is a "break" command, which may be either when
an erase or break command within the vplot file is processed or when a new
file is opened. The 0-level group consists of everything in the plot frame
excepting erases and breaks. Erases and breaks lie between groups. Groups
generated by the begin-end group commands in the vplot file are numbered
from 1 on up.

The external integer "group_number" gives the number of currently
open groups.