Function atanh 

pub fn atanh(x: f64) -> f64

Returns the hyperbolic arc (also called inverse) tangent of x, returning a value in radians. Use it to get the angle from an angle’s tangent in hyperbolic space if x is between -1 and 1 (non-inclusive).

In mathematics, the inverse hyperbolic tangent is only defined for -1 < x < 1 in the real set, so values equal or lower to -1 for x return negative @GDScript.INF and values equal or higher than 1 return positive @GDScript.INF in order to prevent [method atanh] from returning @GDScript.NAN.

var a = atanh(0.9) # Returns 1.47221948958322
tanh(a) # Returns 0.9

var b = atanh(-2) # Returns -inf
tanh(b) # Returns -1