pgplsql - accessing rows from below or above.

Issue:
------------------------------
I am trying to get a plpgsql to access a value above (or below) the
current row itself.

Example:
------------------------------
select
node
from
(values('once'),('twice'))
hello(node);

This produces
once
twice

doing this query:
select
node,
lag(node,1) over (rows between unbounded preceding and unbounded following)
from
(values('once'),('twice'))
hello(node);

produces:
once
twice once

That is the effect I am trying to get in pgplsql.

Example of attempt at a solution:
------------------------------
I have no problem writing pgplsql to affect data on same rows, but I don't
seem to comprehend or see in the documentation as far as I can tell you to
do this.
(as an aside, why I want this I so I can do comparisons of say 80 rows vs
the current row and evaluate to something. I have this working in excel vba
but am trying to put it into the db due to volume of data etc...)
I thought maybe I could do a query in pgplsql and move its value by the
number of rows - but this seems quote verbose and the query never seems to
end.

create or replace function valueabovereal(node text) returns text language
plpgsql as $$
declare
t text;
total_count integer;
u integer;
begin
total_count := (
select
count(*)
from
(values('once'),('twice'))
hello(node)
)
;
u := 1;
while u <= total_count loop
t := (
select
upon.node--,
--row_number
from
(select
lag(hello.node,1) over (rows between unbounded preceding and unbounded
following) as node
from
(values('once'),('twice'))
hello(node)
limit 1 offset u
) as upon

)
;
end loop;
return t;
end;
$$

and the query:

select
node,
valueabovereal(node)
from
(values('once'),('twice'))
hello(node);

Does anyone have any pointers or tutorials that show how to access outside
of your current row in pgplsql?