http://www.openbsd.org/faq/pf/macros.html
"Lists
A list allows the specification of multiple similar criteria within a rule.
For example, multiple protocols, port numbers, addresses, etc. So, instead of
writing one filter rule for each IP address that needs to be blocked, one rule
can be written by specifying the IP addresses in a list. Lists are defined by
specifying items within { } brackets.
When pfctl(8) encounters a list during loading of the ruleset, it creates
multiple rules, one for each item in the list."
