We are in great debt to Markus Triska, Technische Universit├Ąt Wien, Austria for putting on display the two attribute variable interfaces on the SWI-Prolog mailing list in 2017.

We are also grateful to Kuniaki Mukai, Keio University, Japan for exchanging code snippets of a prototype SAT solver and encouraging comments.

Finally we would like to thank Jan Wielemaker, Vrije Universiteit Amsterdam for patiently answering questions about the SWI-Prolog system.