  A scenario for yielding of granular matter is presented by considering the
ensemble of force networks for a given contact network and applied shear stress
$\tau$. As $\tau$ is increased, the probability distribution of contact forces
becomes highly anisotropic, the difference between average contact forces along
minor and major axis grows, and the allowed networks span a shrinking subspace
of all force-networks. Eventually, contacts start to break, and at the yielding
shear stress, the packing becomes effectively isostatic. The size of the
allowed subspace exhibits simple scaling properties, which lead to a prediction
of the yield stress for packings of arbitrary contact number.
