dovecot-sieve: require "mailbox" fails (Unbuntu 16.04)

[Karim 'Kasi Mir' Senoucci]

Hello everyone, 

I'm moving some old procmail recipes to sieve, but having problems with
it. The problem lies with the mailbox extension, which I'm trying to use
as proposed in an example in the Dovecot wiki: 

> require ["variables","date","fileinto","mailbox"];
> 
> # Extract date info
> if currentdate :matches "year" "*" { set "year" "${1}"; }
> if currentdate :matches "month" "*" { set "month" "${1}"; }
> 
> # Archive Dovecot mailing list items by year and month.
> # Create folder when it does not exist.
> if header :is "list-id" "dovecot.dovecot.org" {
> fileinto :create "INBOX.Lists.${year}.${month}.dovecot";
> }

I'm trying a similar rule (different condition, different mail folder),
but cannot seen to get it to work. I always get the errors: 

> .DOVECOT: LINE 16: ERROR: UNEXPECTED '}' FOUND AT (THE PRESUMED) END OF FILE. 
> .DOVECOT: ERROR: PARSE FAILED.

I'm using the standard dovecot packages from Ubuntu 16.04: 

> II DOVECOT-CORE 1:2.2.22-1UBUNTU2 AMD64 SECURE POP3/IMAP SERVER - CORE FILES 
> II DOVECOT-IMAPD 1:2.2.22-1UBUNTU2 AMD64 SECURE POP3/IMAP SERVER - IMAP DAEMON 
> II DOVECOT-MANAGESIEVED 1:2.2.22-1UBUNTU2 AMD64 SECURE POP3/IMAP SERVER - MANAGE SIEVE SERVER 
> II DOVECOT-SIEVE 1:2.2.22-1UBUNTU2 AMD64 SECURE POP3/IMAP SERVER - SIEVE FILTERS SUPPORT

I was under the impression that these sieve extensions, including
"mailbox", are builtin into dovecot/pigeonhole, or at least packaged
into dovecot-sieve. Is that not the case? How can I get the example
working under Ubuntu linux 16.04? Do I need to add or reconfigure
something by hand? 

Thanks in advance for any help on the subject. 

Greetings
Kasi Mir