α = 0 rad (slider from 0 to 6.283185307179586, step 0.0017453292519943296)
β = 0 rad (slider from 0 to 6.283185307179586, step 0.0017453292519943296)
A = (4, -2)
B = (8, 2)
c = Circle(A, B)
r = Segment(A, B)
C: A + (r; α)
D: C + (r / 4.0; 2.0 * β)
a = Segment(C, D)