# File lib/chef/provider/package/dpkg.rb, line 33
        def define_resource_requirements
          super
          requirements.assert(:install) do |a| 
            a.assertion{ not @new_resource.source.nil? }
            a.failure_message Chef::Exceptions::Package, "Source for package #{@new_resource.name} required for action install"
          end

          # TODO this was originally written for any action in which .source is provided
          # but would it make more sense to only look at source if the action is :install?
          requirements.assert(:all_actions) do |a| 
            a.assertion { @source_exists }
            a.failure_message Chef::Exceptions::Package, "Package #{@new_resource.name} not found: #{@new_resource.source}"
            a.whyrun "Assuming it would have been previously downloaded."
          end
        end