  We suggest that the pseudo-rapidity cut dependence of diffractive
deep-inelastic scattering events at HERA may provide a sensitive test of models
of diffraction. A comparison with the experimental cross section shows that the
Donnachie-Landshoff model and a simple two-gluon exchange model of the pomeron
model are disfavoured. However a model with a direct coupling of the pomeron to
quarks is viable for a harder quark--pomeron form factor, as is a model based
on the leading-twist operator contribution. We also consider a direct-coupling
scalar pomeron model. We comment on the implications of these results for the
determination of the partonic structure of the pomeron.
