use ieee.numeric_std.all;


These vendor specific and derived libraries are replaced by a 'new' IEEE standard package:
Defines operations on arrays of Std_logic,
Defines 2 new types. (also in the earlier libraries)
type UNSIGNED is array (NATURAL range <>) of STD_LOGIC;
type SIGNED   is array (NATURAL range <>) of STD_LOGIC;

Provides the following overloaded operators on

combinations of 
UNSIGNED and NATURAL
SIGNED and INTEGER

+ - * / rem mod > >= < <= = /=

Not all of these will be supported by synthesis tools.