Sending Arguments To Xmlhttp.onreadystatechange Event

Hw do I send arguments to an onreadystatefunction when calling a document via xmlhttprequest object. For example,

I want to do something like

function getDoc(url)
xmlhttp=new XMLHttpRequest();
xmlhttp.onreadystatechange=writeHTML(id);
xmlhttp.open("GET",url,true);
xmlhttp.send(null);
}

function writeHTML(id){
if (xmlhttp.readyState==4)
{
// if "OK"
if (xmlhttp.status==200)
{
document.getElementById(id).innerHTML = xmlhttp.responseText;
}
}