Class Polyhedron is defined in polyhed.web.
It is derived from Solid_Faced using public derivation. It is
intended for use as a base class for specific types of polyhedra.
Currently, the classes Tetrahedron, Dodecahedron,
Icosahedron, and Trunc_Octahedron (truncated octahedron)
are derived from Polyhedron.
There is a great deal of work left to do on the polyhedra.