A = (-1, 1)
B = (1, 1)
C = (0, -1)
t = 0.2 (slider from 0 to 1, step 0.1)
M: t * A + (1 - t) * C
N: t * C + (1 - t) * B
P: t * M + (1 - t) * N
lieu1 = Locus(P, t)
a = Line(M, N)