  A two-boson realization of the second hamiltonian structure for the KP
hierarchy has recently appeared in the literature. Furthermore, it has been
claimed that this is also a realization of the hierarchy itself. This is
surprising because it would mean that the dynamics of the KP hierarchy---which
in its usual formulation requires an infinite number of fields---can be
described with only two. The purpose of this short note is to point out the
almost obvious fact that the hierarchy described by the two bosons is not the
KP hierarchy but rather a reduction thereof---one which is moreover
incompatible with the reduction to the KdV-type subhierarchies.
