# File lib/nokogiri/xml/schema.rb, line 45
      def validate thing
        if thing.is_a?(Nokogiri::XML::Document) 
          validate_document(thing) 
        elsif File.file?(thing)
          validate_file(thing)
        else
          raise ArgumentError, "Must provide Nokogiri::Xml::Document or the name of an existing file"
        end
      end