By Magri's theorem the bi-Hamiltonian structure of Plebanski's second
heavenly equation proves that (anti)-self-dual gravity is a completely
integrable system in four dimensions.Comment: This is a contribution to the Proc. of workshop on Geometric Aspects
of Integrable Systems (July 17-19, 2006; Coimbra, Portugal), published in
SIGMA (Symmetry, Integrability and Geometry: Methods and Applications) at
http://www.emis.de/journals/SIGMA