  The target measure $\mu$ is the distribution of a random vector in a box
$\cB$, a Cartesian product of bounded intervals. The Gibbs sampler is a Markov
chain with invariant measure $\mu$. A ``coupling from the past'' construction
of the Gibbs sampler is used to show ergodicity of the dynamics and to
perfectly simulate $\mu$. An algorithm to sample vectors with multinormal
distribution truncated to $\cB$ is then implemented.
