plgpsql and transactions

In a BEFORE INSERT trigger, depending on input values, I need to lock
a table and do a few selects. Of course, the "lock table" isn't much
use if not currently in a transaction.

So my question is this: can I tell if I'm inside a transaction or
not and issue a BEGIN if not. And then also set a flag so that after
the INSERT I can detect that I issued a BEGIN and do a COMMIT?

Or, maybe better is to just throw an exception if not already inside a
transaction.

BTW -- it seems odd to me that you can issue a lock table outside of
an explicit begin/commit and not get a warning. When would issuing a
lock table outside an explicit transaction be of any use?

--
Bill Moseley
moseley(at)hank(dot)org