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add_filter( 'pre_get_posts', 'tgm_io_cpt_search' );
/**
 * This function modifies the main WordPress query to include an array of 
 * post types instead of the default 'post' post type.
 *
 * @param object $query  The original query.
 * @return object $query The amended query.
 */
function tgm_io_cpt_search( $query ) {	
  if ( $query->is_search ) {
	  $query->set( 'post_type', array( 'post', 'doc' ) );
  }    
  return $query;    
}