function fill_circle(r,x0,y0,fill_style)
% fill_circle  Draw a solid circle (a solid disk) in the (x,y) plane
%
% Synopsis:  fill_circle(r)
%            fill_circle(r,x0)
%            fill_circle(r,x0,y0)
%
% Input:  r = radius of the circle
%         x0,y0 = x and y coordinates of the center of the circle
%                 Default:  x0 = 0,  y0 = 0;

if nargin<2, x0 = 0;  end
if nargin<3, y0 = 0;  end
if nargin<4, fill_style = 'b';  end

t = linspace(0,2*pi);
x = x0 + r*cos(t);
y = y0 + r*sin(t);
fill(x,y,fill_style)