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   (list w-1 w-2 w-3 w-4))
 </lang>
+=={{Header|SETL}}==
+<lang SETL>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;</lang>
+===Alternate version (uses [http://www.setl-lang.org/wiki/index.php/Backtracking backtracking])===
+<lang SETL>program amb;
+sets := unstr('[{the that a} {frog elephant thing} ' +
+              '{walked treaded grows} {slowly quickly}]');
+print(amb(sets));
+proc amb(sets);
+  sentence := [];
+  (for words in sets)
+    if exists word in words |
+              ok and (sentence = [] or
+                      word(1) = (reverse sentence(#sentence))(1)) then
+      sentence with:= word;
+    else
+      fail;
+    end if;
+  end;
+  return sentence;
+end proc;
+end program;</lang>