function v = exam4f(x,y,Z)
ylag1 = Z(:,1);
ylag2 = Z(:,2);
v = zeros(3,1);

v(1) = -y(1)*ylag1(2) + ylag2(2);
v(2) =  y(1)*ylag1(2) - y(2);
v(3) =  y(2) - ylag2(2);