A = (-2, 0)
B = (2, 0)
a = 4.1 (slider from 0 to 6, step 0.1)
c = Circle(A, a)
d = Circle(B, 6 - a)
C = Intersect(c, d, 1)
b = Segment(A, C)
e = Segment(C, B)
D = Intersect(c, d, 2)
f = Segment(B, D)
g = Segment(D, A)