  Vertex operators, which are disguised Darboux maps, transform solutions of
the KP equation into new ones. In this paper, we show that the bi-infinite
sequence obtained by Darboux transforming an arbitrary KP solution recursively
forward and backwards, yields a solution to the discrete KP-hierarchy. The
latter is a KP hierarchy where the continuous space x-variable gets replaced by
a discrete n-variable. The fact that these sequences satisfy the discrete KP
hierarchy is tantamount to certain bilinear relations connecting the
consecutive KP solutions in the sequence. At the Grassmannian level, these
relations are equivalent to a very simple fact, which is the nesting of the
associated infinite-dimensional planes (flag).
  It turns out that many new and old systems lead to such discrete
(semi-infinite) solutions, like sequences of soliton solutions, with more and
more solitons, sequences of Calogero-Moser systems, having more and more
particles, band matrices, etc... ; this will be developped in another paper. In
this paper, as an other example, we show that the q-KP hierarchy maps, via a
kind of Fourier transform, into the discrete KP hierarchy, enabling us to write
down a very large class of solutions to the q-KP hierarchy.
