  In this paper we prove a new chain rule formula for the distributional
derivative of the composite function $v(x)=B(x,u(x))$, where $u:]a,b[\to\R^d$
has bounded variation, $B(x,\cdot)$ is continuously differentiable and
$B(\cdot,u)$ has bounded variation. We propose an application of this formula
in order to deal in an intrinsic way with the discontinuous flux appearing in
conservation laws in one space variable.
