You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: spm_tf2ss.m
    path of file: ./repos/bspm/thirdparty/spm8/toolbox/Neural_Models
    
    the code of the file until where you have to start completion: function [a,b,c,d] = tf2ss(num, den)
%TF2SS  Transfer function to state-space conversion.

    
function [a,b,c,d] = tf2ss(num, den)
%TF2SS  Transfer function to state-space conversion.
%   [A,B,C,D] = TF2SS(NUM,DEN)  calculates the state-space 
%   representation:
%       .
%       x = Ax + Bu
%       y = Cx + Du
%
%   of the system:
%               NUM(s) 
%       H(s) = --------
%               DEN(s)
%
%   from a single input.  Vector DEN must contain the coefficients of
%   the denominator in descend