The problem is that any time you want to interface to code outside of that library, you run into the same "convert vector to list" problem the OP was talking ab

The problem is that any time you want to interface to code outside of that library, you run into the same "convert vector to list" problem the OP was talking about. The solution really is to cram down Rhodes' proposal into every implementation's throat, so that all existing code can start handling new sequence types.