policy.rb

class TravelDocumentPolicy
  
  def create?
    user.agent? || user.superagent?
  end
  
  def update?
    create?
  end
  
end

#### Or
class ApplicationPolicy
  
  private
  
  def self.permit_ah_agents_to(*actions)
    actions.each do |action|
      define_method("#{ action }?") do
        user.agent? || user.superagent?
      end
    end
  end

end

class TravelDocumentPolicy
  
  permit_ah_agents_to :create, :update

end