def atan2h(x, y):
    """
    Hyperbolic arctangent
    >>> arctanh(z) = 1/2 Log((1+z)/(1-z))

    real:
        the following must be true: |x1| > |x2| and x2 ≠ 0.

    complex:
        x1 = a + bi
        x2 = b + di
        a = b = 0 and (sign of c) = (sign of d):
             the result is 0.
        a = b = 0 and (sign of c) ≠ (sign of d):
             the result is π.
        c = d = 0 and (sign of a) = (sign of b):
             the result is π/2.
        c = d = 0 and (sign of a) ≠ (sign of b):
             the result is −π/2
    """
    raise NotImplementedError()
