# 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