function circle(a0,b0,radius)
%
%
% displaying a circle.
%
x_first=a0+radius;
y_first=b0;
for j=1:360
  x_next=a0+radius*cos(2*j*pi/360);
  y_next=b0+radius*sin(2*j*pi/360);
  line([x_first x_next], [y_first y_next]);
  x_first=x_next;
  y_first=y_next;
end % j