How to fire triggers just on "top" level DML

We've been running for about ten years on a framework which fires
triggers similar to database triggers in a Java tier close to the
database, and we're now trying to convert these to actual PostgreSQL
database triggers.  Our biggest hitch at the moment is that we
defined a class of triggers we called "top" triggers, which only
fire from DML submitted by the application, not from DML issued by
other triggers.

One significant use of this is to block direct modification of
summary data (either selected columns or entire tables) which are
supposed to be trigger maintained.  It's not immediately obvious how
to accomplish this within PostgreSQL, although I'm probably missing
something.  We're not tied to any particular methodology -- a
TG_DEPTH variable, if it existed, would do fine, for example.

Any suggestions?

-Kevin