How do I access the current row from a trigger?

Hi,

I have a table as:
create table sections (    id           serial not null primary key,    parent_id    references sections(id)
);

there is only one row where the parent_id is NULL, and that is the root 
section, all others refers to a section. Now I want to create a trigger, 
so that whenever I delete a section I can delete all sections that has 
the deleted section as parent, which in turn will run the triggers for 
the deleted sections. Thus I will be able to clear an entire branch with 
only one SQL command from the client.

consider the content:
id          parent_id
0           NULL
1           0
2           1
3           2

and when I run the command
delete from sections where id=1;

I want this to trigger a function which will do
delete from sections where parent_id=( 'id' field in the current row[1] )

which in turn will trigger the same function to do
delete from sections where parent_id=( 'id' field in the current row[2] )

is there any way to do this in PostgreSQL? I couldn't find a way of 
refering to the current row in a trigger in the postgresql reference.

Sincerely,
/Magnus Sjöstrand