  \\
% \nonumber q_3 &=& \psi
% \end{eqnarray}
% </p>
% <p>where \(x\) and \(y\) are the coordinates of the CG of the vehicle.
% \(\psi\) is the yaw angle of the vehicle.</p> 
% </html>

% symfun
q_t     = [x_t y_t PSI_t];
dq_t    = diff(q_t,t);
ddq_t   = diff(diff(q_t,t));
% sym
q       = [ x   y   PSI   ];
dq      = [ dx  dy  dPSI  ];
ddq     = [ ddx ddy ddPSI ];

%% Alternative velocity related variables 
% <html>
% <p>However, in many occasions it is more convenient to use the states \(v_{\rm T}\) e \(\