function modificationtest()
clc;

sys{1}  = tf(         [1],          [1]);
sys{2}  = tf(         [1],      [1,  1]);
sys{3}  = tf(     [1,  1],          [1]);
sys{4}  = tf(     [1,  1],      [1,  3]);
sys{5}  = tf(     [1,  3],      [1,  1]);
sys{6}  = tf(         [1], [1, 0.1, 10]);
sys{7}  = tf([1,   1, 10],          [1]);
sys{8}  = tf([1,   3, 10], [1,   1, 10]);
sys{9}  = tf([1,   1, 10], [1,   3, 10]);

sys{10} = tf([1, 0.1,  1], [1,   1,  1]) * tf([1, 100, 1e8], [1, 300, 1e8]);
sys{11} = tf([1, 0.1,  1], [1,   1,  1]) * tf([1, 100, 1e8], [1, 300, 1e8]) * tf([1], [1, 0]);

sys{12} = tf(         [1],      [1,  0]);
sys{13} = tf(     [1,  0],      [1,100]);

sys{14}  = sys{6} * sys{6};
sys{15}  = sys{6} * sys{6} * sys{6};

sys{16}  = tf([1,  0.01, 10], [1, 0.01, 15]);
sys{17}  = sys{16} * tf([1,  0.01, 20], [1, 0.01, 25]);

sys{18}  = tf([1,  0.001, 150000], [1, 0.001, 160000]);
sys{19}  = sys{18} * tf([1,  0.001, 170000], [1, 0.001, 180000]);

sys{20}  = sys{17} * sys{19};

sys{21} = tf([1], [1, 0, 1]);
sys{22} = tf([1, 0, 100], [1, 0, 10000]);

sys{23} = tf([1,  -1, 10],          [1]);
sys{24} = tf([1,  -3, 10], [1,  -1, 10]);
sys{25} = tf([1,  -1, 10], [1,  -3, 10]);

for idxx = 1 : length(sys)
  nyquist(sys{idxx});
  saveas(gcf, ["nyq", num2str(idxx, "%02d")], "png");
  close;
  
  zp = [zero(sys{idxx}); pole(sys{idxx})];
  zp = zp( imag(zp) >= 0 );
  zp = zp( abs(zp) > 0 );
  
  for jdxx = 1 : length(zp)
    bode(sys{idxx}, {abs(zp(jdxx))*0.9, abs(zp(jdxx))*1.1});
    saveas(gcf, ["bode ", num2str(idxx, "%02d"),...
                 " ",     num2str(jdxx, "%02d")], "png");
  end
  close;
end

end
