def __add_abcd_counts(x, y):
    """
    Adds two tuples. For example.

    :math:`x + y = (x_a + y_a, x_b + y_b, x_c + y_c, x_d + y_d)`

    :param x: Tuple (a, b, c, d).
    :param y: Tuple (a, b, c, d).
    :return: Tuple (a, b, c, d).
    """
    return x[0] + y[0], x[1] + y[1], x[2] + y[2], x[3] + y[3]