code: def all_roots_valid?
        if acts_as_nested_set_options[:scope]
          roots.group_by{|record| scope_field_names.collect{|field| record.send(field.to_sym)}}.all? do |scope, grouped_roots|
            each_root_valid?(grouped_roots)
          end
        else
          each_root_valid?(roots)
        end
      end
docs: Wrapper for each_root_valid? that can deal with scope
   Warning: Very expensive!  Do not use unless you know what you are doing.