The p_rules array is an associative array with the key being the DESTINATION fieldname
and the values being the SOURCE fieldname (without Fs).
E.g.
$p_rules = array('a' => 'a', 'b' => 'title', 'd' => 'body');
p_rules is verified elsewhere (see article_types/merge3.php).
If we are doing an actual merge, all that happens is that we rename the Type in Articles
from SrcType to DestType and run the merge (p_rules) on the XSrcTable entries and move
them over to XDestType. Merged articles have the same ArticleNumber as their originals.