What's the most concise cross-browser way to access an <iframe> element's window and document?

I'm trying to figure out the best way to access an <iframe> element's window and document properties from a parent page. The <iframe> may be created via JavaScript or accessed via a reference stored in an object property or a variable, so, if I understand correctly, that rules out the use of document.frames.

I've seen this done a number of ways, but I'm unsure about the best approach. Given an <iframe> created in this way:

var iframe = document.createElement('iframe');
document.getElementsByTagName('body')[0].appendChild(iframe);

I'm currently using this to access the document, and it seems to work OK across the major browsers:

var doc = iframe.contentWindow || iframe.contentDocument;
if (doc.document) {
  doc = doc.document;
}

I've also see this approach:

var iframe = document.getElementById('my_iframe');

iframe = (iframe.contentWindow) ? iframe.contentWindow :
         (iframe.contentDocument.document) ? iframe.contentDocument.document :
         iframe.contentDocument;

iframe.document.open();
iframe.document.write('Hello World!');
iframe.document.close();

That confuses me, since it seems that if iframe.contentDocument.document is defined, you're going to end up trying to access iframe.contentDocument.document.document.

There's also this:

var frame_ref = document.getElementsByTagName('iframe')[0];

var iframe_doc = frame_ref.contentWindow ? frame_ref.contentWindow.document :
    frame_ref.contentDocument;

In the end, I guess I'm confused as to which properties hold which properties, whether contentDocument is equivalent to document or whether there is such a property as contentDocument.document, etc.

Can anyone point me to an accur