The codice_33 attribute, supported by several major search engines, controls whether search engine spiders are allowed to index a page, or not, and whether they should follow links from a page, or not. The attribute can contain one or more comma-separate values. The codice_35 value prevents a page from being indexed, and codice_36 prevents links from being crawled. Other values recognized by one or more search engines can influence how the engine indexes pages, and how those pages appear on the search results. These include codice_37, which instructs a search engine not to store an archived copy of the page, and codice_38, which asks that the search engine not include a snippet from the page along with the page's listing in search results.