function col = rcol(x,m1,m2) ;
% produces a concatenation of each step of the restriction of a system of
% resistors on SG (in Y-circuits)

col = x ;

n = m1 - m2 ;
if(n <= 0)
  return
end

m = m1 ;
for i = 1:n
  x = restrict(x,m,m-1) ;
  col = [col ; x] ;
  m = m-1 ; 
end
