That’s all true (and thanks for pointing out Clark which I didn’t know) but I think the ISO standard is probably the last place a novice programmer will look to for help in understanding what “predicate” means.
In general, there is a lot of subtext in Prolog that is not explained in any one source and that it takes a lot of effort to put together from disparate sources. It reminds me a bit of my frustration with R, the language, and how its peculiarities are often explained by “well, it’s a front-end for S”. Oh, good. What is S?
Edit: Rather, the turn of phrase I’ve often seen is that R “was built on top of S” (not that it’s a front-end for it). Apologies for the terminological blunder. In any case, the point is that if you don’t understand why something is the way it is in R, the answer is often that it was done that weay in S.