[Feature Request]: Assertion for `id` arg in `a_freq_j()`.
This feature request asks for an assertion to be added to the `a_freq_j()` function to validate the `id` argument. Currently, missing or incorrect `id` values lead to uninformative error messages, making it difficult for users to diagnose problems. Adding this assertion would improve the user experience by providing clearer feedback when the `id` argument is not properly specified.