+ The parser checks whether all cross-references that are used are defined
+ (if not, an error is produced) and whether all cross-references that are
+defined are used (if not, a warning is produced). It also checks whether
+the type of the cross-reference is the same on definition and use (if not,
+an error is produced). The first two checks are done at the end of
+the parsing, because cross-references can be defined after their usage in
+GEDCOM.<br>
+<br>
+The following functions are available to manipulate xref_value objects:<br>
+<blockquote><code>struct xref_value* <b>gedcom_get_by_xref</b> (const char *key)</code><br>
+ <blockquote> Retrieve an xref_value by its key. Returns <code>NULL</code> if the given key isn't a valid cross-reference key (see below) or isn't used.<br>
+ </blockquote>
+ <code>struct xref_value* <b>gedcom_add_xref</b> (Xref_type type, const char* key, Gedcom_ctxt object)</code> <br>
+ <blockquote>Add an xref_value of the given <code>type</code> (see list above), with the given <code>key</code>, to the given <code>object</code>, with a use count equal to 0. Returns the new xref_value if success. Returns <code>NULL</code> in one of the following cases:<br>
+ <ul>
+ <li>the key isn't a valid cross-reference key (see below)</li>
+ <li>there is already an xref_value with the same key</li>
+ <li>there was a memory allocation error</li>
+ </ul>
+ </blockquote>
+ <code>int <b>gedcom_delete_xref</b> (const char* key)<br>
+ </code>
+ <blockquote>Delete the xref_value corresponding to the given key. Returns 0 if success. Returns 1 in one of the following cases:<br>
+ <ul>
+ <li>the key isn't a valid cross-reference key (see below)</li>
+ <li>there is no xref_value with the given key</li>
+ <li>the xref_value is still in use, i.e. its use count is not 0 (see <code>gedcom_link_xref</code> and <code>gedcom_unlink_xref</code> below)</li>
+ </ul>
+ </blockquote>
+ <code>struct xref_value* <b>gedcom_link_xref</b> (Xref_type type, const char* key)<br>
+struct xref_value* <b>gedcom_unlink_xref</b> (Xref_type type, const char* key)</code><br>
+ <blockquote>Declare
+the xref_value corresponding to the given key as being used/no longer used
+(linked to or unlinked) as the given type. The use of these functions
+is not mandatory, but it can aid in spotting places in the code where xref_value
+objects are deleted while they are still referenced.<br>
+ <br>
+Returns the xref_value object if success, and its use count is incremented/decremented. Returns <code>NULL</code> in one of the following cases:<br>
+ <ul>
+ <li>the key isn't a valid cross-reference key (see below)</li>
+ <li>there is no xref_value with the given key</li>
+ <li>the xref_value was previously added as another type than the type provided here<br>
+ </li>
+ </ul>
+ </blockquote>
+</blockquote>
+<blockquote>
+ </blockquote>
+
+A cross-reference key must be a string of maximum 22 characters, of the following format:<br>
+<ul>
+ <li>an at sign ('@')</li>
+ <li>followed by an alphanumeric character (A-Z, a-z, 0-9 or underscore)</li>
+ <li>followed by zero or more characters, which can be any character except an at sign</li>
+ <li>terminated by an at sign ('@')</li>
+</ul>
+An example would thus be: "@This is an xref_val@".<br>
+