`arg/3` is well designed to fail simply in case of out of arity

Nice to know
1

I have found that arg/3 simply fails when the index is out of range. I changed related codes. I believed that it raises out-of-range exception when it is. Here are related new and old style of using arg/3, new one is more natural in that index is increasing, but the old one is decreasing. Also new one can be used without knowing the arity of the term. I like it.

% NEW style
% ?- unify_args(1, f(A, B, C), a).
%@ A = B, B = C, C = a.
unify_args(I, X, A):- arg(I, X, A), !,
	J is I + 1,
	unify_args(J, X, A).
unify_args(_, _, _).


% OLD style
% ?- old_unify_args(3, f(A, B, C), a).
%@ A = B, B = C, C = a.
	
old_unify_args(0, _, _):-!.
old_unify_args(I, X, A):- arg(I, X, A), !,
	J is I - 1,
	old_unify_args(J, X, A).