    # File lib/chef/provider/group.rb, line 61
          def define_resource_requirements
            requirements.assert(:modify) do |a|
              a.assertion { @group_exists }
              a.failure_message(Chef::Exceptions::Group, "Cannot modify #{@new_resource} - group does not exist!")
              a.whyrun("Group #{@new_resource} does not exist. Unless it would have been created earlier in this run, this attempt to modify it would fail.")
            end
          end