t = 0.3 (slider from 0 to 1, step 0.1)
A = (2, 1)
B = (1, 2)
C = (4, 3)
f = Segment(A, B)
g = Segment(B, C)
I = Dilate(B, t, A)
J = Dilate(C, t, B)
M = Dilate(J, t, I)
lieu1 = Locus(M, t)
h = Segment(J, I)