  Coherence effects are important in the description of collisions with
extended objects as nuclei. At fixed target energies and small x_F, the
coherence length of the fluctuation containing the $c\bar c$ is small and the
usual nuclear absorption model is valid. However, at higher energies and/or x_F
the nucleus is seen as a whole by the fluctuation. In this case, the total, not
the absorptive, $c\bar c-N$ cross section controls the suppression and also
shadowing of gluons appears. We propose that the growth of the coherence length
can explain the x_F-dependence of present experimental data. For this, we need
a ratio of absorptive over total $c\bar c-N$ cross section of 0.2.
