Hertling, Peter and
Selivanov, Victor. 2014.
“Complexity Issues for Preorders on Finite Labeled Forests.” in
Logic, Computation, Hierarchies, edited by Vasco
Brattka, Hannes
Diener, and Dieter
Spreen, pp. 165–190.
Ontos Mathematical Logic n. 4. Berlin: de Gruyter.