Function file 

pub fn file(
    caller: &Caller,
    to: &MailAddress,
    body: &str,
    filing: &Filing,
) -> Result<Filed>

File body from caller in the mailbox of to, on this machine’s mail root whatever machine to names (mail for another machine waits here until the watch loop has carried it there). A wake is requested only by the filing that wrote the message, so a repeat never delivers it twice.