function newA = MakeSwitch(p,q,r,s,A)
 
% Performs possible ``switch'' on the matrix A
 
newA = 0; 
[m n] = size(A); 
 
if (p==r | p==s | q==r | q==s | p==q | r==s) return; end
if (p<1 | p>n | q<1 | q>n | r<1 | r>m | s<1 | s>m) return; end
for i = 1:m
     if (A(i,p) ~= A(i,q)) return; end 
end 
for i = 1:n   
     if (A(r,i) ~= A(s,i)) return; end
end 
if (A(p,r) == 1 & A(p,s) == 0 & A(q,r) == 0 & A(q,s) == 1) 
else 
     if (A(p,r) == 0 & A(p,s) == 1 & A(q,r) == 1 & A(q,s) == 0)
     else return; end 
end
 
newA = A; 
if (A(p,r) == 1)
     newA(p,r) = 0; newA(p,s) = 1; 
     newA(q,r) = 1; newA(q,s) = 0; 
else 
     newA(p,r) = 1; newA(p,s) = 0; 
     newA(q,r) = 0; newA(q,s) = 1;
end