def to_fractional(self, cartesian):
return np.dot(cartesian, self.reciprocal)<EOL>
Convert Cartesian to fractional coordinates

           Argument:
            | ``cartesian``  --  Can be a numpy array with shape (3, ) or with shape
                                 (N, 3).

           The return value has the same shape as the argument. This function is
           the inverse of to_cartesian.