Debug a somethnig going weird in someone elses nodeapi presave

This could be prettier. Do not judge. Especially could be prettier using an IDE.

<?php
/**
 * Invoke a hook_nodeapi() operation in all modules.
 *
 * @param &$node
 *   A node object.
 * @param $op
 *   A string containing the name of the nodeapi operation.
 * @param $a3, $a4
 *   Arguments to pass on to the hook, after the $node and $op arguments.
 * @return
 *   The returned value of the invoked hooks.
 */
function node_invoke_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
  $return = array();
  foreach (module_implements('nodeapi') as $name) {
if ($op == 'presave') {
// @debug
$v = (array)$node;
drupal_set_message($v['uid']);
}
    $function = $name .'_nodeapi';
if ($op == 'presave') drupal_set_message('blame: ' . $name);   
    $result = $function($node, $op, $a3, $a4);
    if (isset($result) && is_array($result)) {
      $return = array_merge($return, $result);
    }
    else if (isset($result)) {
      $return[] = $result;
    }
if ($op == 'presave') {
// @debug
$v = (array)$node;
drupal_set_message($v['uid']);
}
  }
  return $return;
}
?>

Do this if things are all right and good at the start of a node_save and are not after nodeapi presave is called.
Where the value you are interested in stops showing up, that's the module that is sabotaging you.

Resolution