@techee yeah, I wasn't worried about VPL, its got an IDE of its own, although IIUC the compiler is available as freeware like VC++ editions.
But I was just interested in why the GNU prolog was not the "most popular"[^1] open prolog, and like you I used @rdipardo as the most prolog savvy person here, so asked him.
[^1]: measurement method unspecified