  Given five points in a three-dimensional euclidean space, one can consider
five tetrahedra, using those points as vertices. We present a pentagon-like
formula containing the product of three volumes of those tetrahedra in its
l.h.s. and the product of the two remaining tetrahedron volumes in its r.h.s.,
as well as the derivative of the "discrete curvature" which arises when we
slightly deform our euclidean space.
