How do I use hook_nodeapi()?
The following code is used because the module is already rendering the CCK field, and wants to avoid that Drupal renders the content of that field.

if (is_array($node-&gt;content['field_svar'])) {
  unset($node-&gt;content['field_svar']);
}


Without using that code, the field content will appear twice: once inside a field set, and once outside that field set.

The last parameters used for hook_nodeapi() are described from the documentation as:


  $a3 
  
  
  For "view", passes in the $teaser parameter from node_view().
  For "validate", passes in the $form parameter from node_validate().
  
  
  $a4
  
  
  For "view", passes in the $page parameter from node_view().
  


It means that, when the operation is "view," $a3 is TRUE when Drupal is displaying the teaser (and FALSE when Drupal is displaying the full node content), and $a4 is TRUE when the node is being displayed alone in a page (and FALSE when the node is being displayed together other nodes in the same page).
To understand the meaning of the different values given to $a4, $a4 is TRUE when Drupal is displaying, for example, the page http://example.com/node/1 (where the same page displays only a node); its value is FALSE when Drupal is visualizing the front page and there are more than one node promoted to the front page.