Can you use an array as a index key?

Seeking clarification, the doc page (https://docs.aws.amazon.com/qldb/latest/developerguide/working.create.html) says:

"Indexes can only be created on a single top-level field. Composite, nested, unique, and function-based indexes are currently not supported."

So does that mean that you can only create an index on simple Ion values? I'd like to use a tuple/list of strings as a key, and while it'd be easy enough to generate a single string from the list, it'd be nice to avoid it if possible.

Thanks!