Is this command outdated?

Hello,

On the Readme page here there is a command which does not work (any more?). It is this one here:

himalaya flag add --flag seen 1:3,5

It also throws errors when I replace the numbers in the example with actual numbers from my mailbox. I also do not understand what those numbers in the example mean exactly. Is it "Add flag seen to messages 1 to 3 and 5" or something else?

Thank you for explaining...