procedure r_sum(u: boolean; var x1,x2,x3: hreal);
begin { r_sum }
  rsum(x1,x2,x3);
  if trunc then begin
    if u then begin
      if x3.si>0 then rtowinfty(x3);
    end
    else if x3.si<0 then rtowinfty(x3);
  end;
end { r_sum };

