Meaning of "protocol !indexer-worker"

Hello,

I'm configuring dovecot 2.2.28. Comparing with previous versions I have 

found now in 10-mail.conf the config:

protocol !indexer-worker { # If folder vsize calculation requires opening more than this many mails from # disk (i.e. mail sizes aren't in cache already), return failure and finish # the calculation via indexer process. Disabled by default. This setting must # be 0 for indexer-worker processes. #mail_vsize_bg_after_count = 0 }

I can see that indexer-worker is the index service in dovecot.

But I don't what the '!' means in front of the service name. Any who 

can explain it to me?

-- Angel L. Mateo Martínez Sección de Telemática Área de Tecnologías de la Información y las Comunicaciones Aplicadas (ATICA) http://www.um.es/atica Tfo: 868889150 Fax: 868888337