sm2des system matrix to descriptor

## Syntax

    Des = sm2des(Sm);

## Arguments

Sm

  polynomial matrix (pencil system matrix)

Des

  descriptor system (`list('des',A,B,C,D,E)`)

## Description

Utility function: converts the system matrix:

    Sm = [-sE + A   B;
         [    C     D]

to descriptor system `Des=list('des',A,B,C,D,E)`).

## See also

- systmat
- sm2ss
- ss2des