  We give a syntactic view of the Sawada-Williams $(\sigma,\tau)$-generation of
permutations. The corresponding sequence of $\sigma-\tau$-operations, of length
$n!-1$ is shown to be highly compressible: it has $O(n^2\log n)$ bit
description. Using this compact description we design fast algorithms for
ranking and unranking permutations.
