function G=feedback(G1,G2)
% feedback - find the overall model of two FOSS objects in feedback connection
%
%    G=feedback(G1,G2)
%
%   G1, G2 - the FOSS objects in the forward and backward paths
%   G - the overall model of the closed-loop system

% Copyright (c) Dingyu Xue, Northeastern University, China
% Last modified 28 March, 2017 
% Last modified 18 May, 2022 
   G1=foss(G1); G2=foss(G2); 
   if length(G1.alpha)>1 || length(G2.alpha)>1, a=0.0001;
   else, a=common_order(G1.alpha,G2.alpha); end
   if a==0
      G=foss([],[],[],G1.d*inv(eye(size(G1.d))+G2.d*G1.d),0);
   elseif a<0.001
      G1=fotf(G1); G2=fotf(G2); G=foss(feedback(G1,G2));
   else
      G1=coss_aug(G1,round(G1.alpha/a));
      G2=coss_aug(G2,round(G2.alpha/a));
      G=foss(feedback(ss_extract(G1),ss_extract(G2))); G.alpha=a;
   end
end