r-Functions with quasi-dynamical spectral parameter

For certain 1 + 1-dimensional classical field theories, whose equations of motion can naturally be written in Lax form by introducing a quasi-dynamical spectral parameter (using sdiff2, the Lie algebra of symplectic diffeomorphisms of a two-dimensional manifold), the previously derived Poisson-commutativity of an infinite set of conserved charges also follows from the existence of an -function, whose functional form is given, and checked explicitly (as well as its Yang-Baxter equation) for an interaction that is exponential (respectively, inverse square) in the spatial derivative of the field.