  The simplest solvable problem of stress transmission through a static
granular material is when the grains are perfectly rigid and have an average
coordination number of $\bar{z}=d+1$. Under these conditions there exists an
analysis of stress which is independent of the analysis of strain and the $d$
equations of force balance $\nabla_{j} \sigma_{ij}({\vec r}) = g_{i}({\vec r})$
have to be supported by $\frac{d(d-1)}{2}$ equations. These equations are of
purely geometric origin. A method of deriving them has been proposed in an
earlier paper. In this paper alternative derivations are discussed and the
problem of the "missing equations" is posed as a geometrical puzzle which has
yet to find a systematic solution as against sensible but fundamentally
arbitrary approaches.
