program amb;

sets := unstr('[{the that a} {frog elephant thing} {walked treaded grows} {slowly quickly}]');

print(amb(sets));

proc amb(sets);
  return amb1([], {}, sets);
end proc;

proc amb1(prev, mbLast, sets);
  if sets = [] then
    return prev;
  else
    words fromb sets;
    if exists word in words |
              (forall last in mbLast |
                      last(#last) = word(1)) and
              (exists sentence in {amb1(prev with word, {word}, sets)} |
                      true) then
      return sentence;
    end if;
  end if;
end proc;

end program;