xbasc()/// @@prerequisite 
function [xdot]=derpol(t,x); 
  xdot=[x(2);-x(1)+(1 - x(1)**2)*x(2)];
endfunction
xf= linspace(-1,1,10);yf= linspace(-1,1,10);
fchamp(derpol,0,xf,yf);  /// \sleftarrow{\normalfont an  \( \R\sp{2}\) vector field (Figure~\ref{f:gr3})} 
xs2ps(0,'fig3'); /// @@prerequisite affectations 
unix(SCI+'/bin/BEpsf fig3'); /// @@prerequisite affectations  /// \sleftarrow{\normalfont calling a Unix script}