function cross(Cx,Cy,v)
% CROSS  draws at position  Cx,Cy  a cross of height 2.5v
%        and width 2*v
Kx = [Cx Cx Cx Cx-v Cx+v];
Ky = [Cy Cy+2.5*v Cy+1.5*v Cy+1.5*v Cy+1.5*v];
plot(Kx,Ky);
plot(Cx,Cy,'o');

