  Let $S_1, \cdots, S_N$ simple finite-dimensional modules of a quantum affine
algebra. We prove that if $S_i\otimes S_j$ is cyclic for any $i < j$ (i.e.
generated by the tensor product of the highest weight vectors), then
$S_1\otimes \cdots \otimes S_N$ is cyclic. The proof is based on the study of
$R$-matrices.
