Functionality to add a filter for descendants and ancestors of a jQuery set.

jquery.ancestry.js

/*
Ancestry - jquery.ancestry.js
As discussed in the jQuery Development Google Group.
Released under the MIT license.

Involved: Michael Geary, Diego Perini, John-David Dalton, John Resig, and Nathan Hammond
Compiled: Nathan Hammond
*/

jQuery.comparePosition = function ( element, context) {
	jQuery.comparePosition = 
	document.documentElement.compareDocumentPosition ?
			function ( element, context ) {
				return !!( element.compareDocumentPosition(context) & 8 );
			}
		: document.documentElement.contains ?
			function ( element, context ) {
				return element  != context && context.contains(element);
			}
		:
			function ( element, context ) {
				for ( ; element != context; element = element.parentNode )
					if ( !element ) return false;
				return true;
			};
	return jQuery.comparePosition( element, context );
}

jQuery.fn.ancestorOf = function ( context ) {
	return this.filter(function() {
		return jQuery.comparePosition( context, this );
	});
};
jQuery.fn.descendantOf = function ( context ) {
	return this.filter(function() {
		return jQuery.comparePosition( this, context );
	});
};

/*
Noted here if you wish to add parallel functionality to $()

if ( selector.nodeType ) {
	// Handle $(DOMElement,	context)
	if ( context &&	!jQuery.comparePosition( selector, context ) ) {
		return jQuery( [] );
	}
	// Handle $(DOMElement)
	this[0]	= selector;
	this.length = 1;
	return this;
}
*/