do :- 
  new(X,picture(ghjkjh)),
  send(X,size,size(300,300)),
  send(X,open),
  new(C,circle(40)),
  send(C,colour,red),
  send(C,fill_pattern,green),
  send(C,position,point(150,150)),
  send(X,display,C),
  new(G,handler(area_enter,message(@prolog,move,C))),
  send(C,recogniser,G).

move(O) :-
  get(O,area,A),
  get(A,x,X),
  get(A,y,Y),
  change(X,XN),
  change(Y,YN),
  send(O,move,point(XN,YN)).

change(X,XN) :-
  X < 50, !,
  XN is floor(X+random(100)).
change(X,XN) :-
  X > 250, !,
  XN is floor(X-random(100)).
change(X,XN) :-
  XN is floor(X + random(100)-50).