Using triggers with non-C functions

Now that I know that triggers can call non-C functions, I would like to
know how non-C functions access information about the triggered row.

I see plpgsql has an interface to the trigger information. Do SQL
functions us OLD/NEW to reference that information, like they do in
rules?

The trigger programmers guide page says PROCEDURE is a C function, so I
will need to correct that:

Procedure

The procedure name is the C function called.

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