Why do some DCG test cases use assertion( Rest == [])?

Question

This answer uses assertion( Rest == [] )?

pharse/3 is

phrase(:DCGBody, ?List, ?Rest)

Since phrase/2 is equivalent to phrase(DCGBody, InputList, []). which is just Rest = [] and in the test case Rest == [], whats the point? What benefit does adding assertion( Rest == [] ) give?

Answer 1

phrase(GB__2, Xs, []) is equivalent to phrase(GB__2, Xs) but it is not necessarily the case that the first solution of phrase(GB__2, Xs0,Xs) will be Xs == [].

Here is a minimal example:

:- set_prolog_flag(double_quotes, chars).

ao1 --> "a" | [].

ao2 --> [] | "a".

While phrase(aoX, "a") just succeeds for both, phrase(aoX, "a", Xs) shows a difference in the sequence of solutions found.

?- phrase(ao1, "a", Xs).
   Xs = "a"
;  Xs = [].
?- phrase(ao2, "a", Xs).
   Xs = []
;  Xs = "a".