Even garden variety ss objects can't have the system matrices changed one by one
sys = rss(4,2,2) % model with four states, two inputs, two outputs
% sys.a = sys.a(1:3,1:3); % attempt to eliminate the fourth state from A, results in an error
Instead, use set to eliminate the fourth state by changing all the matrices at once:
set(sys,'a',sys.a(1:3,1:3),'b',sys.b(1:3,:),'c',sys.c(:,1:3));
sys
Maybe a similar approach will work for a mechss object.
