Backup with lines
If you draw a geometry but don't have saved any room, the backup file does not save anything. If the program crashes, everything will be lost. It could be very useful, if the backup will also save lines (walls or doors), which are not (yet) used for building a room.
Maybe like this:
<geometry (...)>
<rooms>
(...)
</rooms>
<transitions>
(...)
</transitions>
</geometry>
<unused>
<polygon caption="wall">
<vertex px="4" py="0"/>
<vertex px="2" py="0"/>
</polygon>
<polygon caption="door">
<vertex px="4" py="0"/>
<vertex px="7" py="0"/>
</polygon>
</unused>