  We show that every subset of SL_2(Z/pZ) grows rapidly when it acts on itself
by the group operation. It follows readily that, for every set of generators A
of SL_2(Z/pZ), every element of SL_2(Z/pZ) can be expressed as a product of at
most O((log p)^c) elements of the union of A and A^{-1}, where c and the
implied constant are absolute.
