Hi Pilou,

> In the same order of idea it's not obvious that the function "Extrude" can't extrude this ribbon face...
> just some parts if they are trimmed...

Well extrusion is something that actually happens to curves. When you extrude a surface what actually happens is all the edge curves of the surface are extruded and then the surface is copied to the end to make a cap.

Extrusion of curves works by translating the curve in a fixed direction.

What direction are you expecting to use for those curves such that they don't cross over and interfere with themselves making a "self-intersecting" result?

- Michael