On a class of 2D integrable lattice equations

We develop a new approach to the classification of integrable equations of the form $$ u_{xy}=f(u, u_x, u_y, triangle_z u triangle_{bar z}u, triangle_{zbar z}u), $$ where $triangle_{ z}$ and $triangle_{bar z}$ are the forward/backward discrete derivatives. The following 2-step classification procedure is proposed: (1) First we require that the dispersionless limit of the equation is integrable, that is, its characteristic variety defines a conformal structure which is Einstein-Weyl on every solution. (2) Secondly, to the candidate equations selected at the previous step we apply the test of Darboux integrability of reductions obtained by imposing suitable cut-off conditions.