  The standard coherence criterion for lower previsions is expressed using an
infinite number of linear constraints. For lower previsions that are
essentially defined on some finite set of gambles on a finite possibility
space, we present a reformulation of this criterion that only uses a finite
number of constraints. Any such lower prevision is coherent if it lies within
the convex polytope defined by these constraints. The vertices of this polytope
are the extreme coherent lower previsions for the given set of gambles. Our
reformulation makes it possible to compute them. We show how this is done and
illustrate the procedure and its results.
