tf2des - transfer function to descriptor
Transfer function to descriptor form: S=list('d',A,B,C,D,E)
  E*xdot = A*x+B*u
  y = C*x + D*u
   
    
    Note that D=0 if the optional parameter flag= "withD" is not given. Otherwise a maximal rank D matrix is returned in the fifth entry of the list S
s=poly(0,'s'); G=[1/(s-1),s;1,2/s^3]; S1=tf2des(G);des2tf(S1) S2=tf2des(G,"withD");des2tf(S2)