A complex number can be described by two real coordinates.
One may use rectangular coordinates and gets z = x+I*y, where
x = creal(z) and y = cimag(z).
Or one may use polar coordinates and gets z = r*cexp(I*a)
where r = cabs(z) is the "radius", the "modulus", the absolute value of z,
and a = carg(z) is the "phase angle", the argument of z.
One has tan(carg(z)) = cimag(z) / creal(z).
