[Gmsh] Get sub surfaces in OpenCASCADE

[chloros]

Hello,

i want to create two physical groups identifying the torus surface and
the cylinder surface in order to apply boundary conditions. In the
attached example the assignment of TORUSSURFACE and CYLINDERSURFACE is
incorrect because i guess BooleanDifference is altering tags/labels. so
my question is how i can keep track of the labels in question when i
use BooleanDifference.

Best regards and thanks in advance!
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SetFactory("OpenCASCADE");

//Volume of cuboid excluding electrodes
CUBOIDELECTRODEVOLUME = 102;
//Surface of cuboid and electrode surface
CUBOIDELECTRODESURFACE = 104;

TORUSSURFACE = 105;
CYLINDERSURFACE = 106;

Mesh.CharacteristicLengthFactor = 0.4;

bwx = 1;
bwy = 1;
bwz = 0.5;
Box(1) = {-0.5*bwx, -0.5*bwy, -0.5*bwz, bwx, bwy, bwz};

//Electrodes
trlo = 0.1;
trhi = 0.25;
tz = 0.1;
Torus(2) = {0, 0, tz, trhi, trlo, 2*Pi};
SL2[] = Boundary{Volume{2};};

For i In {0:#SL2[]-1}
   Printf("SL2: %g",SL2[i]);
EndFor

BooleanDifference(50) = { Volume{1}; Delete; }{ Volume{2}; Delete; };

cdia = 0.9;
cD = 0.05;
cgap = 0.2;
Cylinder(3) = {0, 0, -0.5*cgap-cD, 0, 0, cD, 0.5*cdia, 2*Pi};
SL3[] = Boundary{Volume{3};};

For i In {0:#SL3[]-1}
   Printf("SL3: %g",SL3[i]);
EndFor

BooleanDifference(51) = { Volume{50}; Delete; }{ Volume{3}; Delete; };

SL51[] = Boundary{Volume{51};};

For i In {0:#SL51[]-1}
   Printf("SL51: %g",SL51[i]);
EndFor

Physical Surface(TORUSSURFACE) = {SL2[]};
Physical Surface(CYLINDERSURFACE) = {SL3[]};
Physical Surface(CUBOIDELECTRODESURFACE) = {SL51[]};
Physical Volume(CUBOIDELECTRODEVOLUME) = {51};