A Wilf--Zeilberger pair (F,G) in the discrete case satisfies the equation
F(n+1,k)−F(n,k)=G(n,k+1)−G(n,k). We present a structural
description of all possible rational Wilf--Zeilberger pairs and their
continuous and mixed analogues.Comment: 17 pages, add the notion of pseudo residues in the differential case,
and some related papers in the reference, ACMES special volume in the Fields
Institute Communications series, 201