This function returns the logical mate of an oid. The logical mate is a singleton that acts as a mate to the first parameter, but is not an end tag in the XML structure. The function can also return the start mate that matches an end mate.

The function returns the null oid if the argument is not a valid oid, an oid that cannot take a logical mate, or if the logical mate is not present in the document. The latter condition means that the document is semantically incomplete or unbalanced.