def compute_average_unit_cell(unit_cell_list):
    """Compute the weighted average unit cell based on a list of

    ((unit cell), nref)

    tuples."""
    w_tot = 0.0
    a_tot = 0.0
    b_tot = 0.0
    c_tot = 0.0
    alpha_tot = 0.0
    beta_tot = 0.0
    gamma_tot = 0.0
    for cell, n_ref in unit_cell_list:
        w_tot += n_ref
        a_tot += cell[0] * n_ref
        b_tot += cell[1] * n_ref
        c_tot += cell[2] * n_ref
        alpha_tot += cell[3] * n_ref
        beta_tot += cell[4] * n_ref
        gamma_tot += cell[5] * n_ref
    return (a_tot / w_tot, b_tot / w_tot, c_tot / w_tot, alpha_tot / w_tot,
        beta_tot / w_tot, gamma_tot / w_tot)
