def normalize(self):
        '''re-normalise a rotation matrix'''
        error = self.a * self.b
        t0 = self.a - (self.b * (0.5 * error))
        t1 = self.b - (self.a * (0.5 * error))
        t2 = t0 % t1
        self.a = t0 * (1.0 / t0.length())
        self.b = t1 * (1.0 / t1.length())
        self.c = t2 * (1.0 / t2.length())