this looks very impressive. forgive me a stupid question as a prolog outsider: is this actually using any logical prolog features, or mostly just the 'function

this looks very impressive. forgive me a stupid question as a prolog outsider: is this actually using any logical prolog features, or mostly just the 'functional' subset of prolog? on a cursory look almost all annotated predicates are either 'det' or 'semidet', and none seem to have ? argument modes, so on the surface it looks like this is mostly straight functional code. am i missing something important? to be sure, prolog has much to recommend it even as a non-logic functional language: great syntax with probably the most flexible grammar out of all non-research languages (e.g. prolog's operator definition facility makes the ilk of haskell's seem like a toy), homoiconicity, etc. but at least my expectation is perhaps for prolog code to make use more of the logical facilities.