  A further class of complex covariant field equations is investigated. These
equations possess several common features: they may be solved, or partially
solved in terms of implicit functional relations, they possess an infinite
number of inequivalent Lagrangians which vanish on the space of solutions of
the equations of motion, they are invariant under linear transformations of the
independent variables, and thus are signature-blind and are consequences of
first order equations of hydrodynamic type.
