  The two-dimensional non-linear sigma model approach to Self-dual Yang-Mills
theory and to Self-dual gravity given by Q-Han Park is an example of the deep
interplay between two and four dimensional physics. In particular, Husain's
two-dimensional chiral model approach to Self-dual gravity is studied. We show
that the infinite hierarchy of conservation laws associated to the Husain model
carries implicitly a hidden infinite Hopf algebra structure.
