function subtree_visibility_changer(subtree_item_div, subtree_control_img, opened_img_src, closed_img_src) {

  if ( subtree_item_div.style.display == 'block' ) {
    subtree_item_div.style.display = 'none';
    subtree_control_img.src = closed_img_src;
  } else {
    subtree_item_div.style.display = 'block';
    subtree_control_img.src = opened_img_src;
  }
}