Warning: Undefined array key 0 in /srv/www/wordpress/wp-content/plugins/code-snippets/php/snippet-ops.php(582) : eval()'d code on line 57

Warning: Attempt to read property "ID_To" on null in /srv/www/wordpress/wp-content/plugins/code-snippets/php/snippet-ops.php(582) : eval()'d code on line 57
Preface - Table of contents n. 4