doveconf syntax question

[A. Schulze]

Hello,

I have one sieve extension enabled:

   /etc/dovecot/conf.d/20-sieve.conf:

   plugin {
     sieve_extensions = +vacation-seconds
   }

now (months later) we want to enable an other extension.
no big deal:

   /etc/dovecot/local.conf:

   plugin {
     sieve_extensions = +editheaders
     ...
   }

This was wrong. We ASSUMED the plus would add the extension to the list.
But at the end only +editheaders was activ.

What's the right way to /add/ sieve_extensions from different config files?
our guess "sieve_extensions = $sieve_extensions +editheaders" do not work:

doveconf -a | grep sieve_extensions
doveconf: Fatal: Error in configuration file /etc/dovecot/local.conf  
line 22: Unknown variable: $sieve_extensions
doveconf: Error: managesieve-login: dump-capability process returned 89
doveconf: Fatal: Error in configuration file /etc/dovecot/local.conf  
line 22: Unknown variable: $sieve_extensions


Andreas