================================================================
Logtalk - Open source object-oriented logic programming language
Release 2.30.7

Copyright (c) 1998-2007 Paulo Moura.  All Rights Reserved.
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% start by loading the loading the example:

| ?- logtalk_load(barriers(loader)).
...


% run example:

| ?- beatles::sing_along.

hello(1)hello(4)hello(2)hello(3)
goodbye(2)goodbye(1)goodbye(3)goodbye(4)

Yes