Was interested in Prolog implementations, with parallel execution (and/or). Sictus has some possibilities (https://sicstus.sics.se/sicstus/docs/3.7.1/html/sicstus_6.html). Also, SWI Prolog has some possibilites: http://www.swi-prolog.org/man/threads.html.

Then, I found some very old links: http://www.sciencedirect.com/science/article/pii/074310669090025Z, https://link.springer.com/article/10.1007/BF03037061, https://www.it.uu.se/research/csd/theses/0025.pdf. Neatly, there is an overview paper (again, old): https://pdfs.semanticscholar.org/8fe5/f71e7e6b2cc91fe7f43c266d16f9ca2fc61d.pdf

This old thread from 2007 has a lot of links: https://groups.google.com/forum/#!topic/comp.lang.prolog/OMkjGMqFLH4

Finally, a MapReduce construct was added to Prolog (not what I was looking for, but still): http://paginas.fe.up.pt/~ee08127/dissertation/pages/AMapReduceConstructForProlog.pdf.

So, in the end, I am confused – while this should be fairly easy to implement (intuitively speaking), there seems a large body of work declaring otherwise.