Движение ромба по кругу - Pascal ABC

uses graphABC;
procedure romb(x,y:integer);
begin
setpencolor(clRed);
moveto(x-20,y);
lineto(x,y-30);
lineto(x+20,y);
lineto(x,y+30);
lineto(x-20,y);
end;
var c,r,x1,y1:integer;
    a:real;
begin
setwindowsize(500,500);
centerwindow;
c:=windowwidth div 2;
r:=150;
a:=0;
lockdrawing;
repeat
clearwindow;
setpencolor(clBlue);
circle(c,c,r);
x1:=c+round(r*cos(a));
y1:=c-round(r*sin(a));
romb(x1,y1);
sleep(10);
a:=a+0.01;
redraw
until false
end.