  We derive an expression for diffractive F_2 structure function which should
be valid at small x for quasi-elastic scattering on a hadron and for
quasi-elastic scattering on a large nucleus. This expression includes multiple
rescatterings of the quark-antiquark pair produced by the virtual photon off
the sources of color charge in a quasi-classical approximation. We find that
there is a relation between such diffractive production and inclusive
processes. In the former, one averages over all colors of sources before
squaring the amplitude, and in the latter one first squares the amplitude and
then averages it in the hadron's or nuclear wave function. We show that in the
limit of a large virtuality of the photon Q^2 the diffractive structure
function becomes linearly proportional to the gluon distribution of the hadron
or nucleus, therefore proving that in this sense diffraction is a leading twist
effect.
