P1,P2,P4,P5 als Quadrat
ergibt einen Kreis

P4 = (0.8569992236358527, 1.0617390099736095)
P5 = (1.2, 0.4)
P1 = (2.2, 0.4)
P2 = (2.373211094933898, 1.3990103358325174)
f = Line(P5, P1)
g = Line(P4, P2)
h = Line(P4, P5)
i = Line(P2, P1)
j = Segment(P4, P1)
k = Segment(P2, P5)
E = Intersect(h, i)
F = Intersect(f, g)
G = Intersect(j, k)
l = Line(E, G)
P3 = Intersect(g, l)
P0 = Intersect(l, f)
m = Segment(P4, P2)
n = Segment(P2, P1)
p = Segment(P1, P5)
q = Segment(P5, P4)
R0(t) = If[0 ≤ t ≤ 0.25, (1 - (4 * t))^(2) / (1 - (4 * t) + (8 * t^(2))), 0]
R1(t) = If[0 ≤ t ≤ 0.5, If[0 ≤ t < 0.25, ((4 * t) * (1 - (3 * t)) / (1 - (4 * t) + (8 * t^(2)))), (1 - (2 * t))^(2) / (1 - (4 * t) + (8 * t^(2)))], 0]
R2(t) = If[0 ≤ t ≤ 0.5, If[0 ≤ t < 0.25, (4 * t^(2) / (1 - (4 * t) + (8 * t^(2)))), (-1 + (8 * t) - (12 * t^(2))) / (1 - (4 * t) + (8 * t^(2)))], 0]
R3(t) = If[0.25 ≤ t ≤ 0.75, If[0.25 ≤ t < 0.5, (1 - (4 * t))^(2) / (1 - (4 * t) + (8 * t^(2))), (3 - (4 * t))^(2) / (5 - (12 * t) + (8 * t^(2)))], 0]
R4(t) = R2(1 - t)
R5(t) = R1(1 - t)
R6(t) = R0(1 - t)
pts: {P0, P1, P2, P3, P4, P5, P0}
px = Sequence(x(Element[pts, i]), i, 1, 7)
py = Sequence(y(Element[pts, i]), i, 1, 7)
fx(t) = (R0(t) * px(1)) + (R1(t) * px(2)) + (R2(t) * px(3)) + (R3(t) * px(4)) + (R4(t) * px(5)) + (R5(t) * px(6)) + (R6(t) * px(7))
fy(t) = (R0(t) * py(1)) + (R1(t) * py(2)) + (R2(t) * py(3)) + (R3(t) * py(4)) + (R4(t) * py(5)) + (R5(t) * py(6)) + (R6(t) * py(7))
kr = CurveCartesian(fx(t), fy(t), t, 0, 1)