Aliasing syntax question

I've now seen the 'unnest' function defined in a few different ways with
regard to aliases.

Substitute the following pairs of values for X and Y in the query below,
and all of them work (in PG 8.3.6, at least):

X   Y
g    g
i     g(i)
g    g(i)
g.i  g(i)

create or replace function unnest(anyarray)
returns setof anyelement as $$
select $1[X] from generate_series(array_lower($1,1),array_upper($1,1)) Y;
$$ language sql;

Please enlighten the unworthy!

Thanks,
Kevin Murphy