PL/pgSQL 'i = i + 1' Syntax

Hellow PostgreSQL hackers,

Quick question. Why does the 'i = i + 1' syntax work in this PL/pgSQL
function?

try=# CREATE OR REPLACE FUNCTION inc_by_two(
try(# upfrom int,
try(# upto int
try(# ) RETURNS SETOF INT AS $$
try$# BEGIN
try$# FOR i IN upfrom..upto LOOP
try$# RETURN NEXT i;
try$# i = i + 1;
try$# END LOOP;
try$# END;
try$# $$ LANGUAGE 'plpgsql';
CREATE FUNCTION
try=# select * from inc_by_two(1,10);
inc_by_two
------------
1
3
5
7
9
(5 rows)

Someone posted a PL/pgSQL function in my blog with this syntax, which
is how I know about it, but I couldn't find it documented anywhere.
Is it a special exception for loop variables, perhaps?

Thanks,

David