  We show that the flatness of a nonlinear discrete-time system can be checked
by computing a unique sequence of involutive distributions. The well-known test
for static feedback linearizability is included as a special case. Since the
computation of the sequence of distributions requires only the solution of
algebraic equations, it allows an efficient implementation in a computer
algebra program. In case of a positive result, a flat output can be obtained by
straightening out the involutive distributions with the Frobenius theorem.
