NU Polymorphisms on Reflexive Digraphs.

We find a set of generators of the variety of reflexive digraphs admitting k - NU polymorphisms. We do this, in spite of the fact that such digraphs do not have finite tree duality, by defining finite duals of infinite trees. As a result of this, we answer a question of Quackenbush, Rival, and Rosenberg, giving a finite family of generators of the variety of finite bounded posets admitting k - NU polymorphisms.