  The exact solution of the asymmetric exclusion problem with N distinct
classes of particles (c = 1,2,...,N), with hierarchical order is presented.
  In this model the particles (size 1) are located at lattice points, and
diffuse with equal asymmetric rates, but particles in a class c do not
distinguish those in the classes c' >c from holes (empty sites). We generalize
and solve exactly this model by considering the molecules in each distinct
class c =1,2,...,N with sizes s_c (s_c = 0,1,2,...), in units of lattice
spacing. The solution is derived by a Bethe ansatz of nested type.
