Function local_dir 

fn local_dir(display: &str, delimiter: Option<&str>) -> String

The Maildir++ directory: a dot, then the levels joined by dots. Characters that a level’s name cannot hold on disk, on any system, are written %XX. An empty level (“INBOX.”, “a//b”, “/” where “/” separates levels) is written %00: joined as it is, it could make “.” or “..”, the account’s Maildir or the folder of every account’s, which removing this folder’s copy would delete.