  Experiments at HERA looking for deep-inelastic electroproduction of neutrons
in the proton fragmentation region are in progress. They are aimed to measure
the pion structure function at small Bjorken $x$. The important condition for
such a study is to establish under what kinematical conditions the dominance of
the pion-pole graph in the process is guaranteed. We analyse other sources of
theleading neutron, in order to figure out the kinematical region where the
one-pion exchange dominates.
