On a class of 2D integrable lattice equations

We develop a new approach to the classification of integrable equations of the form $$u_{xy}=f(u,u_x,u_y,{\mathrm{△}}_{z}u{\mathrm{△}}_{\bar{z}}u,{\mathrm{△}}_{z\bar{z}}u),$$ where ${\mathrm{△}}_z$ and ${\mathrm{△}}_{\bar{z}}$ 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.