Each element of the 'face' array is an array of indices
into the 'vertex' array for the (co-planar) points of a
face of the polyhedron, ordered such that an adjacent pair
(and the first and last) are (the indices of) the end-points
of an edge, and the direction of a cycle produces by the
right-hand rule an outward pointing normal to the face.
A Requirement, as part of the Monitors algorithm for generating
guesses in the N digit number game, keeps track of a set of available digits
and the number of digits that must be picked from that set, and provides
a check that those constraints are satisfied.