Why in SWI Prolog, I get the result with `|` sign?

I have the following code to group sequences of equivalent objects together:

pack([], []).
pack([X], [X]).

pack([H, T|TS], [H|TR]):-
    H \= T,
    pack([T|TS], TR).

pack([H, H|HS], [[H|TFR]|TR]):-
    pack([H|HS], [TFR|TR]).

My input is:

pack([1,1,1,1,2,3,3,1,1,4,5,5,5,5], X).

And as a result I get:

[[1,1,1|1],2,[3|3],[1|1],4,[5,5,5|5]]

Have you noticed | sign before the last element in each list? I do not know why it appears and how to fix it. Any ideas?