  We define a volume function on configurations of four points in the 3-sphere
which is invariant under the action of PU(2,1), the automorphism group of the
CR structure defined on the sphere by its embedding in complex 2-space. We show
that the volume function, constructed using appropriate combinations of the
dilogathm function of Bloch-Wigner, satisfies a five term relation in a more
general context which includes at the same time CR and real hyperbolic
geometry.
