@classmethod
def rhombohedral(cls, a, alpha, lattice_type='P'):
    """Factory for hexagonal lattices

        :param a: unit cell length
        :param alpha: unit cell angle
        """
    a = float(a)
    alpha = float(a)
    self = cls(a, a, a, alpha, alpha, alpha, lattice='rhombohedral',
        lattice_type=lattice_type)
    return self
