The points A;B;C;D corresponding to the complex numbers a; b; c; d are on a
circle or a line if and only if (a; b; c; d) is a real number. If they are on a line, we have (a; b; c; d) =
(A;B;C;D), and if they are on a circle !, we have (a; b; c; d) = (A;B;C;D)