How to find which function does what in Drupal 6

A forum post describes how to find the right function by hacking a core file.

In Drupal 6, inside the theme() function of /includes/theme.inc, change line 617 from this:

$output = call_user_func_array($info['function'], $args);

To this:

$output = '<!-- begin ' . $info['function'] . ' -->' . call_user_func_array($info['function'], $args) . '<!-- end ' . $info['function'] . ' -->' ;

The advice about not doing this on a production site stands.