function calculates the complex arc hyperpolic cosine of
If y = cacosh(z), then z = ccosh(y).
The imaginary part of
is chosen in the interval [-pi,pi].
The real part of
is chosen nonnegative.
cacosh(z) = (0.5) * clog((1 + z) / (1 - z))
These functions first appeared in glibc in version 2.1.