create function first_nonnull( one anyelement, two anyelement )
    returns anyelement
as $$
begin
    return one;
end
$$
    language plpgsql strict;

create function last_nonnull( one anyelement, two anyelement )
    returns anyelement
as $$
begin
    return two;
end
$$
    language plpgsql strict;

create aggregate first_nonnull( anyelement )
(
    stype = anyelement
  , sfunc = first_nonnull
);

create aggregate last_nonnull( anyelement )
(
    stype = anyelement
  , sfunc = last_nonnull
);


create function first( head anyarray, tail anyelement )
    returns anyarray
as $$
begin
    if head is null or head = '{}' then
        return ARRAY[tail];
    end if;
    return head;
end
$$
    language plpgsql;

create function last( head anyarray, tail anyelement )
    returns anyarray
as $$
begin
    return ARRAY[tail];
end
$$
    language plpgsql;

create function first( head anyarray )
    returns anyelement
as $$
begin
    return head[1];
end
$$
    language plpgsql;

create aggregate first( anyelement )
(
    stype = anyarray
  , initcond = '{}'
  , sfunc = first
  , finalfunc = first
);

create aggregate last( anyelement )
(
    stype = anyarray
  , initcond = '{}'
  , sfunc = last
  , finalfunc = first
);