  In the paper we define a "volume" for simplicial complexes of flag
tetrahedra. This generalizes and unifies the classical volume of hyperbolic
manifolds and the volume of CR tetrahedra complexes. We describe when this
volume belongs to the Bloch group. In doing so, we recover and generalize
results of Neumann-Zagier, Neumann, and Kabaya. Our approach is very related to
the work of Fock and Goncharov.
