##### Synopsis

    #include <math.h>
    float atanhf(float val)

##### Description

atanhf returns the inverse hyperbolic tangent of val.

##### Fast math library behavior

If |x| ? 1, errno is set to EDOM and atanhf returns HUGE_VALF.

##### IEC 60559 math library behavior

If |val| > 1 atanhf returns NaN with signal.
If val is NaN, atanhf returns that NaN with no signal.
If val is 1, atanhf returns ? with signal.
If val is -1, atanhf returns -? with signal.

##### Portability

atanhf conforms to ISO/IEC 9899:1999 (C99).