If the wall_position (starting point) and wall_direction stay the same when you mirror a wall then you need something that will tell you which side the reference line is on (inside/outside) or whether the wall has been flipped on the reference line.
I don't see anything it the GDL manual that can tell you this.
This might be a good question to ask in the forum at GDG Center.
http://gdl.graphisoft.com/forums/
Graphisoft seem to pay a little more attention to questions there.
Barry.
One of the forum moderators.
Versions 6.5 to 27
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