atan2l
extern long double atan2l(long double y, long double x);
__imag__ result = M_LOG10El * atan2l (__imag__ x, __real__ x);
__imag__ result = atan2l (__imag__ x, __real__ x);
return atan2l(cimagl(z), creall(z));
t = atan2l(2.0L * x, a) * 0.5L;