getting at sequence previous/next values

[Louis-David Mitterrand <vindex+lists-pgsql-sql(at)apartia(dot)org>]

Hello,

I've got a table that looks basically like:

	id_show         | serial
	show_name       | text 
	show_type       | text                        
	created_on      | timestamp without time zone 

When looking at a row with an id_show of value n, I'd like to have an 
easy way of knowing the preceding and next values of id_show in the 
sequence (which might have holes).

Is there some convenient way to get that info, or must I do a full-blown 
select?

Thanks,