proc DrawTree(X1, Y1, X2, Y2, Depth);
int  X1, Y1, X2, Y2, Depth;
int  X3, Y3, X4, Y4, X5, Y5, DX, DY, Color;
[if Depth < 7 then
   [DX:= X2 - X1;  DY:= Y1 - Y2;
    X3:= X2 - DY;  Y3:= Y2 - DX;
    X4:= X1 - DY;  Y4:= Y1 - DX;
    X5:= X4 + (DX-DY)/2;  Y5:= Y4 - (DX+DY)/2;
    Color:= $2A + Depth;
    Move(X1, Y1);
    Line(X2, Y2, Color);  Line(X3, Y3, Color);
    Line(X4, Y4, Color);  Line(X1, Y1, Color);
    DrawTree(X4, Y4, X5, Y5, Depth+1);
    DrawTree(X5, Y5, X3, Y3, Depth+1);
   ];
];

def ScrW=320, ScrH=200;
[SetVid($13);
DrawTree(9*ScrW/20, 3*ScrH/4, 11*ScrW/20, 3*ScrH/4, 0);
]