def cannot(action = nil, subject = nil, *attributes_and_conditions, &block)
      add_rule(Rule.new(false, action, subject, *attributes_and_conditions, &block))
    end
def cannot(action = nil, subject = nil, *attributes_and_conditions, &block)
      add_rule(Rule.new(false, action, subject, *attributes_and_conditions, &block))
    end
Defines an ability which cannot be done. Accepts the same arguments as "can".

   can :read, :all
   cannot :read, Comment

 A block can be passed just like "can", however if the logic is complex it is recommended
 to use the "can" method.

   cannot :read, Product do |product|
     product.invisible?
   end