des2tf descriptor to transfer function conversion

## Syntax

                 S = des2tf(sl)
                [Bfs, Bis, chis] = des2tf(sl)

## Arguments

sl

  list (linear system in descriptor form)

Bfs, Bis

  two polynomial matrices

chis

  polynomial

S

  rational matrix

## Description

Given the linear system in descriptor form i.e. `Sl=list('des',A,B,C,D,E)`, `des2tf` converts `sl` into its transfer function representation:

    S=C*(s*E-A)^(-1)*B+D

Called with 3 outputs arguments `des2tf` returns `Bfs` and `Bis` two polynomial matrices, and `chis` polynomial such that:

    S=Bfs/chis - Bis

`chis` is the determinant of `(s*E-A)` (up to a xcative constant);

## Examples

    s=poly(0,'s');
    G=[1/(s+1),s;1+s^2,3*s^3];
    Descrip=tf2des(G);Tf1=des2tf(Descrip)
    Descrip2=tf2des(G,"withD");Tf2=des2tf(Descrip2)
    [A,B,C,D,E]=Descrip2(2:6);Tf3=C*inv(s*E-A)*B+D

## See also

- glever
- pol2des
- tf2des
- ss2tf
- des2ss
- rowshuff