Function file_name 

fn file_name(name: Option<&str>, index: usize) -> String

A name safe to write in a folder, on every system, since what is saved may be synced to another: no path, no control characters, none of the characters Windows forbids ("report.pdf:x" would write a hidden stream there), no dot or space at the end, and no device’s name (“nul.txt”); the extension kept.