It seems essential to me that the library be able to join using multiple columns as the join key. I don't know if the underlying Trie makes that simpler.

The simplest version might simply add a column holding a product (maybe [v]?) of the key-columns to each Frame, then join the new Frames on that new column, then remove that column in the result. That's how I'd do it from outside the library. And that doesn't seem horribly inefficient. But I'm imagining there's a better way?