@dpiponi to answer my own question, it looks like it might be specific to MicroPROLOG, I found a discussion of it here: https://faculty.nps.edu/ncrowe/book/ae.html Though MicroPROLOG also appears to be more of a family of languages than a single language -- this book describes a clearly-related but definitely distinct dialect using prefix notation: https://worldofspectrum.org/pub/sinclair/games-info/m/Micro-PROLOGPrimer.pdf