<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _fast_normalise(self):
 """Normalise the object to a unit quaternion using a fast approximation method if appropriate. Object is guaranteed to be a quaternion of approximately unit length after calling this operation UNLESS the object is equivalent to Quaternion(0) """

        if not self.is_unit():
            mag_squared = np.dot(self.q, self.q)
            if (mag_squared == 0):
                return
            if (abs(1.0 - mag_squared) < 2.107342e-08):
                mag =  ((1.0 + mag_squared) / 2.0) # More efficient. Pade approximation valid if error is small
            else:
                mag =  sqrt(mag_squared) # Error is too big, take the performance hit to calculate the square root properly

            self.q = self.q / mag