double __far atanh(double x); (C99)
Returns the arc hyperbolic tangent of x.
If x is not inside the range [-1, +1], returns a Not-a-Number (NaN) and sets macro EDOM to global variable errno.
If x is -1, returns -HUGE_VAL and sets macro ERANGE to global variable errno.
If x is 1, returns HUGE_VAL and sets macro ERANGE to global variable errno.
This function calculates the arc hyperbolic tangent of x. Specify the angle in radian.