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/**
 * Add category support for pages
 */
function add_category_and_tag_support_for_pages() {
  // Add tag metabox to page
  register_taxonomy_for_object_type('post_tag', 'page');
  // Add category metabox to page
  register_taxonomy_for_object_type('category', 'page');
}
// Add to the admin_init hook of your theme functions.php file
add_action( 'init', 'add_category_and_tag_support_for_pages' );