meaning of asymmetric on help page for intersect

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of 
davidr at rhotrading.com
Sent: Tuesday, January 13, 2009 10:56 AM
To: Juliet Hannah; r-help at r-project.org
Subject: Re: [R] meaning of asymmetric on help page for intersect

There is also a symmetric set difference = union(setdiff(x, 
y), setdiff(y, x)) denoted x \Delta y

-- David


-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org]
On Behalf Of Juliet Hannah
Sent: Tuesday, January 13, 2009 9:41 AM
To: r-help at r-project.org
Subject: [R] meaning of asymmetric on help page for intersect

Sorry if this is a silly question. What does asymmetric refer 
to from ?intersect, and are there any aspects of the result 
that may be different from expected (reason for exclamation point) ?

As toy data, if needed, here are the examples from the help page.

(x <- c(sort(sample(1:20, 9)),NA))
(y <- c(sort(sample(3:23, 7)),NA))

Thanks,

Juliet