[Gmsh] gmsh some definitions are not clear

[Krishna Mohan Gundu]

Dear Christophe Geuzaine,

You have done a wonderful job with mesh generation software and its
documentation. Couple of things are not well addressed in the
documentation and it took me a while to understand. I hope including
these will save some time for the others.

1. Surface Loop definition: while -ve signs in the line loop
definitions are obvious for creating a surface, the -ve signs in the
surface loop definitions are not obvious. You should say somewhere in
the manual that the line loops should have an inward nomal into the
volume(surface loop) being created, if not use a -ve sign.

2. Define hole: it looks like that a hole may not have a coplanar
surface with the volume. I see that this was addressed in the forum.
It was not clear to me for a while, how to create a such a 'hole'.
Here I am attaching a simple exercise I have done and may be
worthwhile to include in the tutorial section.
test2.geo: A simple cube
test3.geo: A pyramid in the x-direction, which has two coplanar
surfaces with the cube
test4.geo: Shows how to 'subtract' test3.geo from test2.geo

I can make them more readable if you want to include them with tutorial section.

regards,
Krishna.
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