go ?=>
  % select which version of amb/2 and joins/2 to test
  member(Amb,[amb,amb2]),
  member(Joins,[joins,join2]),
  println([amb=Amb,joins=Joins]),
  test_amb(amb,joins, Word1,Word2,Word3,Word4),
  println([Word1, Word2, Word3, Word4]),
  nl,
  fail, % get other solutions
  nl.
go => true.

% Test a combination of amb and joins
test_amb(Amb,Joins, Word1,Word2,Word3,Word4) =>
  call(Amb, Word1, ["the","that","a"]),
  call(Amb, Word2, ["frog","elephant","thing"]),
  call(Amb, Word3, ["walked","treaded","grows"]),
  call(Amb, Word4, ["slowly","quickly"]),
  call(Joins, Word1, Word2),
  call(Joins, Word2, Word3),
  call(Joins, Word3, Word4).

% Based on the Prolog solution.
amb(E, [E|_]).
amb(E, [_|ES]) :- amb(E, ES).

joins(Left, Right) =>
  append(_, [T], Left),
  append([R], _, Right),
  ( T != R -> amb(_, [])
  ; true ).

% Another approach, using member/2 for
% generating the words.
amb2([],[]).
amb2(Word,Words) :- member(Word,Words).

joins2(Word1,Word2) :- Word1.last() = Word2.first().