nth3(N) = cc(N,"th"), tween(N) => true.
nth3(N) = cc(N,"st"), N mod 10 = 1 => true.
nth3(N) = cc(N,"nd"), N mod 10 = 2 => true.
nth3(N) = cc(N,"rd"), N mod 10 = 3 => true.
nth3(N) = cc(N,"th")  => true.
% helper function
cc(N,Th) = N.to_string() ++ Th.