A 3-dimensional topological entity: a region of space bounded by
closed shells, each connected, which never cross. A point is inside the
solid when it is inside an odd number of them, so a shell is a lump of
its own or a void in one by where it lies, not by its place in
shells: a boolean cutting a bar in two leaves two lumps, one leaving
a sphere’s room inside a block a lump and a void. A void’s faces point
into it, out of the material, so the volume it encloses is negative.