Change WordPress RSS feed output (i.e. exclude a category)

gistfile1.php

<?php
function pg_change_rss_query( $query ) {
  if ( ! $query->is_feed )
    return $query;

  // Excludes posts in the staff category from the current feed
  $query->set( 'category_name', 'staff' );

  return $query;
}
add_filter( 'pre_get_posts', 'pg_change_rss_query' );