 is NR-by-NR, B1 is NR-by-M, C1 is P-by-NR, and the other
  submatrices have appropriate dimensions.

  The matrices of the reduced order system (Ar,Br,Cr,Dr) are
  computed according to the following residualization formulas:
                             -1                               -1
     Ar = A11 + A12*(g*I-A22)  *A21 ,  Br = B1 + A12*(g*I-A22)  *B2
                           -1                               -1
     Cr = C1 + C2*(g*I-A22)  *A21   ,  Dr = D + C2*(g*I-A22)  *B2

  where g = 0 if DICO 