var runtil = /Until$/,
	rparentsprev = /^(?:parents|prev(?:Until|All))/,
	isSimple = /^.[^:#\[\.,]*$/,
	rneedsContext = jQuery.expr.match.needsContext,
	// methods guaranteed to produce a unique set when starting from a unique set
	guaranteedUnique = {
		children: true,
		contents: true,
		next: true,
		prev: true
	};

jQuery.fn.extend({
	find: function( selector ) {
		var self, matched, i,
			l = this.length;

if ( typeof selector !== "string" ) {
			self = this;
			return this.pushStack( jQuery( selector ).filter(function() {
				for ( i = 0; i < l; i++ ) {
					if ( jQuery.contains( self[ i ], this ) ) {
						return true;
					}
				}
			}) );
		}

matched = [];
		for ( i = 0; i < l; i++ ) {
			jQuery.find( selector, this[ i ], matched );
		}

// Needed because $( selector, context ) becomes $( context ).find( selector )
		matched = this.pushStack( l > 1 ? jQuery.unique( matched ) : matched );
		matched.selector = ( this.selector ? this.selector + " " : "" ) + selector;
		return matched;
	},

has: function( target ) {
		var targets = jQuery( target, this ),
			l = targets.length;

return this.filter(function() {
			var i = 0;
			for ( ; i < l; i++ ) {
				if ( jQuery.contains( this, targets[i] ) ) {
					return true;
				}
			}
		});
	},

not: function( selector ) {
		return this.pushStack( winnow(this, selector, false) );
	},

filter: function( selector ) {
		return this.pushStack( winnow(this, selector, true) );
	},

is: function( selector ) {
		return !!selector && (
			typeof selector === "string" ?
				// If this is a positional/relative selector, check membership in the returned set
				// so $("p:first").is("p:last") won't return true for a doc with two "p".
				rneedsContext.test( selector ) ?
					jQuery( selector, this.context ).index( this[ 0 ] ) >= 0 :
					jQuery.filter( selector, this ).length > 0 :
				this.filter( selector ).length > 0 );
	},

closest: function( selectors, context ) {
		var cur,
			i = 0,
			l = this.length,
			matched = [],
			pos = ( rneedsContext.test( selectors ) || typeof selectors !== "string" ) ?
				jQuery( selectors, context || this.context ) :
				0;

for ( ; i < l; i++ ) {
			for ( cur = this[i]; cur && cur !== context; cur = cur.parentNode ) {
				// Always skip document fragments
				if ( cur.nodeType < 11 && (pos ?
					pos.index(cur) > -1 :

// Don't pass non-elements to Sizzle
					cur.nodeType === 1 &&
						jQuery.find.matchesSelector(cur, selectors)) ) {

cur = matched.push( cur );
					break;
				}
			}
		}

return this.pushStack( matched.length > 1 ? jQuery.unique( matched ) : matched );
	},

// Determine the position of an element within
	// the matched set of elements
	index: function( elem ) {

// No argument, return index in parent
		if ( !elem ) {
			return ( this[ 0 ] && this[ 0 ].parentNode ) ? this.first().prevAll().length : -1;
		}

// index in selector
		if ( typeof elem === "string" ) {
			return core_indexOf.call( jQuery( elem ), this[ 0 ] );
		}

// Locate the position of the desired element
		return core_indexOf.call( this,

// If it receives a jQuery object, the first element is used
			elem.jquery ? elem[ 0 ] : elem
		);
	},

add: function( selector, context ) {
		var set = typeof selector === "string" ?
				jQuery( selector, context ) :
				jQuery.makeArray( selector && selector.nodeType ? [ selector ] : selector ),
			all = jQuery.merge( this.get(), set );

return this.pushStack( jQuery.unique(all) );
	},

addBack: function( selector ) {
		return this.add( selector == null ?
			this.prevObject : this.prevObject.filter(selector)
		);
	}
});

function sibling( cur, dir ) {
	while ( (cur = cur[dir]) && cur.nodeType !== 1 ) {}

return cur;
}

jQuery.each({
	parent: function( elem ) {
		var parent = elem.parentNode;
		return parent && parent.nodeType !== 11 ? parent : null;
	},
	parents: function( elem ) {
		return jQuery.dir( elem, "parentNode" );
	},
	parentsUntil: function( elem, i, until ) {
		return jQuery.dir( elem, "parentNode", until );
	},
	next: function( elem ) {
		return sibling( elem, "nextSibling" );
	},
	prev: function( elem ) {
		return sibling( elem, "previousSibling" );
	},
	nextAll: function( elem ) {
		return jQuery.dir( elem, "nextSibling" );
	},
	prevAll: function( elem ) {
		return jQuery.dir( elem, "previousSibling" );
	},
	nextUntil: function( elem, i, until ) {
		return jQuery.dir( elem, "nextSibling", until );
	},
	prevUntil: function( elem, i, until ) {
		return jQuery.dir( elem, "previousSibling", until );
	},
	siblings: function( elem ) {
		return jQuery.sibling( ( elem.parentNode || {} ).firstChild, elem );
	},
	children: function( elem ) {
		return jQuery.sibling( elem.firstChild );
	},
	contents: function( elem ) {
		return jQuery.nodeName( elem, "iframe" ) ?
			elem.contentDocument || elem.contentWindow.document :
			jQuery.merge( [], elem.childNodes );
	}
}, function( name, fn ) {
	jQuery.fn[ name ] = function( until, selector ) {
		var matched = jQuery.map( this, fn, until );

if ( !runtil.test( name ) ) {
			selector = until;
		}

if ( selector && typeof selector === "string" ) {
			matched = jQuery.filter( selector, matched );
		}

if ( this.length > 1 ) {
			if ( !guaranteedUnique[ name ] ) {
				jQuery.unique( matched );
			}

if ( rparentsprev.test( name ) ) {
				matched.reverse();
			}
		}

return this.pushStack( matched );
	};
});

jQuery.extend({
	filter: function( expr, elems, not ) {
		if ( not ) {
			expr = ":not(" + expr + ")";
		}

return elems.length === 1 ?
			jQuery.find.matchesSelector( elems[ 0 ], expr ) ? [ elems[ 0 ] ] : [] :
			jQuery.find.matches( expr, elems );
	},

dir: function( elem, dir, until ) {
		var matched = [],
			truncate = until !== undefined;

while ( (elem = elem[ dir ]) && elem.nodeType !== 9 ) {
			if ( elem.nodeType === 1 ) {
				if ( truncate && jQuery( elem ).is( until ) ) {
					break;
				}
				matched.push( elem );
			}
		}
		return matched;
	},

sibling: function( n, elem ) {
		var matched = [];

for ( ; n; n = n.nextSibling ) {
			if ( n.nodeType === 1 && n !== elem ) {
				matched.push( n );
			}
		}

return matched;
	}
});

// Implement the identical functionality for filter and not
function winnow( elements, qualifier, keep ) {

// Can't pass null or undefined to indexOf in Firefox 4
	// Set to 0 to skip string check
	qualifier = qualifier || 0;

var filtered;

if ( jQuery.isFunction( qualifier ) ) {
		return jQuery.grep(elements, function( elem, i ) {
			var retVal = !!qualifier.call( elem, i, elem );
			return retVal === keep;
		});
	}

if ( qualifier.nodeType ) {
		return jQuery.grep(elements, function( elem ) {
			return ( elem === qualifier ) === keep;
		});
	}

if ( typeof qualifier === "string" ) {
		filtered = jQuery.grep(elements, function( elem ) {
			return elem.nodeType === 1;
		});

if ( isSimple.test( qualifier ) ) {
			return jQuery.filter( qualifier, filtered, !keep );
		}

qualifier = jQuery.filter( qualifier, filtered );
	}

return jQuery.grep(elements, function( elem ) {
		return ( core_indexOf.call( qualifier, elem ) >= 0 ) === keep;
	});
}

src/traversing.js

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