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create function first_nonnull( one anyelement, two anyelement )
    returns anyelement
...
  , sfunc = last
  , finalfunc = first
);

select first(v), first_nonnull(v), last(v), last_nonnull(v)
from ( select NULL::text as v union all
...
select first(v), first_nonnull(v), last(v), last_nonnull(v)
from ( select 'one'::text as v union all
    select 'two' as v union all select NULL as v ) as q;

 first | first_nonnull | last | last_nonnull
-------+---------------+------+--------------
       | one           | two  | two
 one   | one           |      | two
(2 rows)