function animatebar (t0,tf,x0);
    % animatebar animates a bar pendulum.
    :
    disp ('Do you want to see the phase portrait?')
    ans = input ('Enter 1 if YES, 0 if NO');
    if ans==0            % see text for description        
        return            % if the input is 0
                    % exit function
    else
               plot (x,......)        % show the phase plot
    end