program amb;

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

words := [amb(words): words in sets];
if exists lWord = words(i), rWord in {words(i+1)} |
          lWord(#lWord) /= rWord(1) then
  fail;
end if;

proc amb(words);
  return arb {word in words | ok};
end proc;

end program;