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I have drawn a SOLID but I cannot select it during the EXTRUDE operation. I can select other objects, such as CIRCLEs and RECTANGLEs.
SOLID is important for drawing irregular polygons, so it should be EXTRUDEable.
You can both extrude and push/pull solid faces (among other operations on them). Extruding a solid, as such, is an undefined operation. Extruding a cube into a tesseract is an intriguing idea, though.
I figured this out.
I have to type REGION and then click on the irregular polygon, the result of SOLID.
Then I can extrude it.
Hmm, confusing... The word 'Solid' can point to (at least) 3 things: '2D' solid (create with the _Solid command), 3D solid, or hatch with a solid hatch pattern.
I assume the OP is referring to '2D' solids.
It seems that BC considers these to be obsolete and offers limited 3D feature for them.