    # File lib/chef/environment.rb, line 126
        def cookbook_versions(arg=nil)
          set_or_return(
            :cookbook_versions,
            arg,
            {
              :kind_of => Hash,
              :callbacks => {
                "should be a valid set of cookbook version requirements" => lambda { |cv| Chef::Environment.validate_cookbook_versions(cv) }
              }
            }
          )
        end