 transformation
  in Step 4 and r elementary transformations in Step 5 (to reduce
  an upper Hessenberg matrix to upper triangular form). These
  special elementary transformations are numerically stable
  in practice.

  In the multi-input multi-output (MIMO) case, the algorithm
  computes each element (i,j) of the transfer function matrix G,
  for i = 1 : P, and for j = 1 : M. For efficiency reasons, Step 1
  is performed once for each value of j (each column of B). The
  matrices Ac and Ao result in Hess