def METHOD_NAME(self):
    """Returns the natural logarithm of the quaternion. 
    (not tested)
    """
    # Init
    norm = self.norm()
    vecNorm = self.x**2 + self.y**2 + self.z**2
    tmp = self.w / norm
    q = Quaternion()
    # Calculate
    q.w = np.METHOD_NAME(norm)
    q.x = np.METHOD_NAME(norm) * self.x * np.arccos(tmp) / vecNorm
    q.y = np.METHOD_NAME(norm) * self.y * np.arccos(tmp) / vecNorm
    q.z = np.METHOD_NAME(norm) * self.z * np.arccos(tmp) / vecNorm
    return q