title: need to use std_logic_Arith with std_logic_signed/unsigned
body: <p>I read in many VHDL file headers :   use  ieee.std_logic_arith.all along with ieee.std_logic_signed.all , i can't see any benefit of this as the latter package(Signed)makes use of the former(Arith) automatically in its implementation. Any clarification why they are used together?</p>
  
  <p>I have the same remark re. ieee.std_logic_unsigned.all
  Thank you </p>