I simpler test case would be possibly one from mathematics.
Showing that the pattern fy 2 yf has only limited very specific use.
Lets say we have a minus and a factorial operator:
:- op(200, fy, -).
:- op(200, yf, !).
:- use_module(library(arithmetic)).
:- arithmetic_function(!/1).
!(0,1) :- !.
!(N,X) :- N > 0, M is N-1, !(M,Y), X is N*Y.
?- X is 10!.
X = 3628800.
?- X is - 10!.
false.
Other Prolog systems might return -3628800, but nevertheless
the advice could be to never use fy and yf with same priority.
If fy and fy with different priority are used the table 6 of ISO
core standard becomes irrelevant and the code gets more portable.