[off-topic] LaTex for [[ ... ]]

[Fergus Henderson]

Sorry for the off-topic post, but I figured someone here would know...
can anyone please tell me how to do the double-square-bracket symbols
that are often used in denotational semantics in LaTex?  It's like
"[[" but a single symbol without the space between the two brackets.
(I checked the Latex companion, but it's not listed there AFAICT.)

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.