As it was for SQL 2000, the SQL 2005 Books Online (BOL) is for me the quickest way to research something.
The Oracle squence is one of the things I miss in SQL Server. The thing that comes close is an IDENTITY column. In your example you seem to generate an unique number for a table. So define a_id as an IDENTITY column (see CREATE TABLE in the BOL) and forget about the trigger.
Note, after rereading this, the IDENTITY column works better/simpler than a sequence
...but anyway, PROPER use of surrogate keys would not require adding prefixes to indicate their location. That should be discouraged. A surrogate key should have no inherent relationship to the data it identifies.
If it's not practically useful, then it's practically useless.