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

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

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

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