I have accepted the flags. Seems the posts stay there greyed? Anyway, debating the ISO standard is off topic. Most of would like SWI-Prolog’s parser to accept all valid ISO text and produce the term dictated by the standard. That is not the discussion. @ridgeworks did a good job trying to make the rules explicit. Handling the fy 1 yfx 2 is clearly not standard compliant and thus a bug. It is just that I happen to have other priorities and will for this only fulfill my duties handling a possibly pull request. I have no clue whether this is simple or hard to fix. The code handling operators is a bit of a mess 
That is typically what happens if you write such a beast without a standard around and a variety of implementations. As just implementing the standard and nothing more will break a lot of existing code this is not really an option either.