for a discrete-time system. The subroutine AB09HD determines for
  the given system (1), the matrices of a reduced order system

       d[z(t)] = Ar*z(t) + Br*u(t)
       yr(t)   = Cr*z(t) + Dr*u(t),                  (2)

  such that

       INFNORM[inv(conj(W))*(G-Gr)] &lt;=
                    (1+HSV(NR+NS-N+1)) / (1-HSV(NR+NS-N+1)) + ...
                    + (1+HSV(NS)) / (1-HSV(NS)) - 1,

  where G and Gr are transfer-function matrices of the systems
  (A,B,C,D) and (Ar,Br,Cr,Dr), respectively, W is th