Cercare il moto retrogrado; quello di questo punto
 non lo e', rispetto al centro.

x0 = -3
vTrasl = 2.2 (slider from 0 to 6, step 0.1)
t = 4.71 (slider from 0 to 5, step 0.01)
xA: x0 + vTrasl * t
R = 2 (slider from 0 to 5, step 0.1)
C: (xA, 2.0)
c = Circle(C, R)
B = Point(c)
vRot = 0.6 (slider from 0 to 4, step 0.1)
β: -1.0 * vRot * t
B' = Rotate(B, β, C)
a = Segment(C, B)
b = Segment(C, B')
α = Angle(B', C, B)