Returns the phase angle of its complex argument.
    This function is supported on OpenVMS Integrity servers and
    Alpha only.
    Format
      #include  <complex.h>
      double carg  (double complex z);
      float cargf  (float complex z);
      long double cargl  (long double complex z);
1 – Argument
 z
    A complex value.
2 – Description
    The carg functions compute the argument (also called phase angle)
    of z, with a branch cut along the negative real axis.
3 – Return Values
    n                  The value of the argument of z, in the
                       interval [-π, +π].