Show/Hide Element?

Alright, I got the basic idea (which is relatively easy):

function show_el(id)
{
var obj = document.getElementById(id);
obj.style.display = "block";
}
function hide_el(id)
{
var obj = document.getElementById(id);
obj.style.display = "none";
}

The problem is, you need 2 different links, one to show the element, one to hide it. What I want is one link that changes depending on whether or not the element is showing: If it's showing, the link will be "Hide Element", if it's not, it'll be "Show Element."

I've seen this done a lot, mainly in expanding/collapsing, which is basically what I'm doing. I imagine it's not all that hard, but I just can't seem to figure it out.