Tirri’s new conceptual development of tree automata approaches them from the point of view of universal algebra. The main technique is the use of “congruences of term algebras which saturate the forests operated on” and using them to construct “a congruence which saturates the product forest.… The preservation of recognizability is a straightforward consequence of those congruence constructions and the Nerode type of congruence characterization for recognizable forests” (from the abstract).