Points A,B, and C are on a line in this order. Points D and E lie on the same side of this line, in such a way that triangles ABD and BCE are equilateral. The segments AE and CD intersect in point S. Prove that ∠ASD=60o.[asy]
unitsize(1.5 cm);pair A, B, C, D, E, S;A = (0,0);
B = (1,0);
C = (2.5,0);
D = dir(60);
E = B + 1.5*dir(60);
S = extension(C,D,A,E);fill(A--B--D--cycle, gray(0.8));
fill(B--C--E--cycle, gray(0.8));
draw(interp(A,C,-0.1)--interp(A,C,1.1));
draw(A--D--B--E--C);
draw(A--E);
draw(C--D);
draw(anglemark(D,S,A,5));dot("A", A, dir(270));
dot("B", B, dir(270));
dot("C", C, dir(270));
dot("D", D, N);
dot("E", E, N);
dot("S", S, N);
[/asy] geometryEquilateral TriangleEquilateralangles