def METHOD_NAME(
    *factors: CovarianceFunction,
) -> Tuple[CovarianceFunction, ...]:
    expanded_factors = []
    for factor in factors:
        if isinstance(factor, ProductCovarianceFunction):
            # pylint: disable="protected-access"
            expanded_factors.extend(factor._factors)
        else:
            expanded_factors.append(factor)
    return tuple(expanded_factors)