[Gmsh] Assigning physical groups to lattice geometry

[Alex Lindsay]

I have a 2D unit cell consisting of 25 plane surfaces (its meshed form 
is attached). I am generating a 3D cuboid mesh which will be about 20x20 
unit cells using Translate, Duplicata, and Extrude. I would like to be 
able to select all the surfaces that compose a face of the cuboid and 
assign them to a physical group. I've done this in the past for a 
different unit cell using "If" and "For" flow control and carefully 
assigning the surfaces generated from Extrude to lists and then setting 
the Physical groups equal to the lists. However, it was challenging to 
ensure that I was capturing the correct surfaces. It would be much nicer 
to say request all the surfaces that lie in a plane. Is such a selection 
criteria possible in gmsh? I would guess not because it's not very 
general, e.g. it would only really be applicable to flat entities. If 
it's not possible, I would appreciate any suggestions for efficient ways 
of assigning elementary entities to physical groups. Maybe my current 
way is the best way...
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