[Gmsh] Defining surfaces

[Steve Daley]

I am a new user to GMSH.  I am having difficulty understanding one of the 
examples.

lc = 0.2;
Point(1) = {0,0,0,lc};
Point(2) = {1,0,0,lc};
Point(6) = {0,0,3,lc};
Line(1) = {1,2};
Line(2) = {2,6};
Line(3) = {6,1};
Line Loop(4) = {2,3,1};
Plane Surface(5) = {4};
Extrude {{0,0,1}, {0,0,0}, Pi/2} { Surface{5}; }
Extrude {{0,0,1}, {0,0,0}, Pi/2} { Surface{17}; }
Extrude {{0,0,1}, {0,0,0}, Pi/2} { Surface{29}; }
Extrude {{0,0,1}, {0,0,0}, Pi/2} { Surface{41}; }

In the example, I can see how Surface{5} is defined and extruded but where 
are Surface {17} {29} and {41} defined.?

Is the fact that the surface numbers are incremented by 12 a function of the 
cone shape ?

I have developed my own example 3D tet mesh and can generate the quarter of 
the shape, (ip to the first of the Extrude lines) but don't know how to do 
the other three quarters  to make up the complete mesh.

Any help gratefully received.

Steve Daley