[Aetae]

function unitedParent(first,second){
  var parents=function(e){var a=[];while(e=e.parentNode)a.push(e);return a}
  var f=parents(first), s=parents(second), fl=f.length, sl=s.length;
  while(f[--fl]&&f[fl]===s[--sl]);
  return f[fl+1]||null
}