function getNextSiblingByNodeName(node_name) {
  var node = this;
  while (node = node.nextSibling) {
    if (node.style.display != 'none') {
      if (node.nodeName == node_name) { break; }
    }
  }
  return node;    
}

function getPreviousSiblingByNodeName(node_name) {
  var node = this;
  while (node = node.previousSibling) {
    if (node.style.display != 'none') {
      if (node.nodeName == node_name) { break; }
    }
  }
  return node;    
}