🚀 go-pugleaf

In article <8210@burdvax.PRC.Unisys.COM> lang@zeta.PRC.Unisys.COM (Francois-Michel Lang) writes:
>> In article <8192@burdvax.PRC.Unisys.COM> lang@zeta.PRC.Unisys.COM (Francois-Michel Lang) writes:
>> >Is it possible to determine (by writing a Prolog program)
>> >which of these two orders is used for given a Prolog system,
>if we assume that the Prolog system one is running
>(1) does indeed use left -> right or right -> left order,
>(2) uses it consistently, and
>(3) can't handle cyclic terms.

Why not simply assume that it uses right->left order consistently and make
the problem _really_ easy?  My earlier reply did not exhaust all the things
that a Prolog system might sensibly do instead.  Imagine a Prolog system
where the compiler replaces all variables in the head by new distinct
variables, and then at the end unifies repeated variables pairwise, e.g.
	p([H|T], L, [H|R]) :-
=>	p([H|T], L, [H'|R]) :- H = H'
If the goal is acyclic, the (simplified) head unification is order-
independent; what matters is the order in which the repeated variables
are handled.  And that might quite plausibly depend on the spelling of
the source variables.

I tell you plainly that Quintus Prolog violates Lang's assumptions.