function y=estop  (rectangle)
% present rectangle with 
% [ p1 p2 p3 p4 p1] 
% [ 0+0i 1+0i 1+2i 0+3i 0]
n = 8;
z = exp(2*pi*i*(0:n)'/n);
plot(z,'-o');
axis square
s = sum(z);
end
