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

        :param a: unit cell length
        :param c: unit cell length
        """
    a = float(a)
    self = cls(a, a, float(c), 90, 90, 120, lattice='hexagonal',
        lattice_type=lattice_type)
    return self
