1. The optional context will be applied on evolving the selector.

2. If the context argument is not given, then at lest use the same document.

Discussion

Here the expected implementation:

Query.fn.add = function( selector, context ) {
    return this.pushStack( jQuery.unique( jQuery.merge(
        this.get(),
        typeof selector === "string" ?
            jQuery( selector, context
                || this.context && this.context.ownerDocument || this.context)
                || this[0] && (this[0].ownerDocument || this[0]) ) :
            jQuery.makeArray( selector )
    )));

};