Calculates the hyperbolic arc tangent of a floating-point number.
long double atanhl(long double d);
d Floating-point number for which hyperbolic arc tangent is to be computed
Normal: Hyperbolic arc tangent of d
Abnormal: Domain error: Returns HUGE_VAL, HUGE_VALF, or HUGE_VALL depending on the function.
Range error: Returns not-a-number.
A domain error occurs for a value of d not in the range [–1, +1]. A range error may occur for a value of d equal to –1 or 1.