  We have completely rewritten the paper, and corrected the proofs. We
construct an exponential map at any point in the (n-1)-skeleton minus the
(n-2)-skeleton of an n-dimensional Riemannian polyhedron. We have added allover
the extra-assumption that the exponential map is totally geodesic at points in
the (n-1)-skeleton minus the (n-2)-skeleton. This condition is automatically
realized for instance if the dimension is one, or if the metric is flat.
