Amb(V)={
	amb(vector(#V,i,vector(#V[i],j,Vec(V[i][j]))),[])
};
amb(V,s)={
	if (#V == 0, return(concat(s)));
	my(v=V[1],U=vecextract(V,2^#V-2),t,final=if(#s,s[#s]));
	if(#s, s = concat(s,[" "]));
	for(i=1,#v,
		if ((#s == 0 | final == v[i][1]),
			t = amb(U, concat(s, v[i]));
			if (t, return(t))
		)
	);
	0
};
Amb([["the","that","a"],["frog","elephant","thing"],["walked","treaded","grows"],["slowly","quickly"]])