  The dressing chain equations for factorizing operators of a spectral problem
are derived. The chain equations itselves yield nonlinear systems which closure
generates solutions of the equations as well as of the nonlinear system if both
operators of the correspondent Hirota bilinearization are covariant with
respect to Darboux transformation which hence defines a symmetry of the
nonlinear system as well as of these closed chains. Examples of Hirota and Nahm
equations are specified.
