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A new sleight of syntax for ya:

 class RubyTalk < Filter
 + to("ruby-talk@ruby-lang.org")
 + cc("ruby-talk@ruby-lang.org")
 - from("common.troll@gifted.net")
 end
 class SelfBlog < Filter
 + to("selfblog@hobix.com")
 + subject("secretpassword")
 def process(msg)
 Blog.add_post(msg)
 end
 end

Young ones, see if you can figure out how it’s done before I hand you the actual raw sprinkles of meta magic dust. Specifically: (1) where are the rules getting stored? and (2) how are the operators actually implemented? Just have a think before the spoilers.

First, the Code

 class Filter
 class Rule
 attr_accessor :field, :terms, :action
 def initialize(f, *a)
 @field = f
 @terms = a
 end
 def -@; @action = :reject end
 def +@; @action = :accept end
 end
 class << self
 attr_accessor :rules
 [:from, :to, :cc, :subject].each do |m|
 define_method(m) do |*a|
 r = Rule.new(m, *a)
 (@rules ||= []) << r
 r
 end
 end
 end
 end

To view the rules for mail going into the RubyTalk mailbox:

 RubyTalk.rules.each do |r|
 puts "[#{r.action}] if #{r.field} matches `#{r.terms}'" 
 end

And, the Answers

1. Where are the rules getting stored? The rules are stored in instance variables inside each class. So, not in a RubyTalk object. In the actual RubyTalk class. Right next to the methods.

2. How are the operators actually implemented? By now, you probably can see just fine. Unary operators on the Rule class. The to and from class methods create Rule classes, which then get charged positively and negatively by the operator.