(* An optical illusion. Drag the points to 
   discover the secret. *)

CONST StickWidth = 25;

PRED IsRect(a, b, c, d) IS 
  (a, b) PARA (d, c) AND 
  (b, c) PARA (a, d) AND 
  Angle.Right(a, b, c) 
END;

UI PointTool(IsRect);

PRED IsHalfRect(a, b, c, d) IS 
  (a, b) PARA (d, c) AND Angle.Right(a, d, c) 
END;

UI PointTool(IsHalfRect);

PROC DrawQuad(a, b, c, d) IS 
  PS.MoveTo(a); 
  PS.LineTo(b); 
  PS.LineTo(c); 
  PS.LineTo(d); 
  PS.Close(); 
  SAVE PS IN 
    PS.SetColor(Color.White); PS.Fill() 
  END; 
  PS.Stroke() 
END;

UI PointTool(DrawQuad);

PROC Cmd0() IS 
  IF 
    VAR 
      q = (-156.77805, -7.584892), r = (142.0091, -17.979002), 
      s = (115.50073, -116.86352), t = (-137.08612, -97.8451), 
      u = (81.03986, 210.1015), v = (-82.55463, 212.37697) 
    IN 
      VAR 
        a ~ (100.47148, -67.31985), b ~ (-64.63778, 216.35509), 
        c ~ (-44.425213, 228.11954), d ~ (120.68404, -55.555393), 
        e ~ (57.49129, 232.22787), f ~ (77.32268, 219.83167), 
        g ~ (-101.41944, -66.11875), h ~ (-121.250824, -53.722538), 
        i ~ (-139.56445, 64.774734), j ~ (28.195253, 56.858265), 
        k ~ (42.207333, 32.784054), l ~ (-140.66684, 41.413757), 
        m ~ (148.65303, 27.760948), n ~ (70.03145, 31.47105), 
        o ~ (56.019363, 55.545265), p ~ (149.75542, 51.12192) 
      IN 
        IsRect(a, b, c, d) AND 
        IsRect(e, f, g, h) AND 
        (c, b) CONG (e, f) AND 
        IsHalfRect(i, j, k, l) AND 
        Geometry.Colinear(a, k, b) AND 
        Geometry.Colinear(a, j, b) AND 
        (c, b) CONG (i, l) AND 
        IsHalfRect(m, n, o, p) AND 
        Geometry.Colinear(d, n, c) AND 
        Geometry.Colinear(d, o, c) AND 
        (m, p) CONG (c, b) AND 
        Geometry.Colinear(i, j, p) AND 
        Geometry.Colinear(i, o, p) AND 
        q = Geometry.Mid(i, l) AND 
        r = Geometry.Mid(p, m) AND 
        s = Geometry.Mid(a, d) AND 
        t = Geometry.Mid(g, h) AND 
        u = Geometry.Mid(e, f) AND 
        v = Geometry.Mid(b, c) AND 
        StickWidth = Geometry.Dist(b, c) -> 
          PS.SetWidth(3); 
          DrawQuad(a, b, c, d); 
          DrawQuad(e, f, g, h); 
          DrawQuad(i, j, k, l); 
          DrawQuad(m, n, o, p) 
      END 
    END 
  FI 
END;