parallel adder package

I want to create a package which contain

• procedure for full adder

• procedure for parallel adder using full adder defined in the same package

but there is some error. Please check the source code

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library ieee;

use ieee.std_logic_1164.all;

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package digital_package is

procedure full_adder(signal a,b,cin:in std_logic;

signal s,c: out std_logic);

procedure parallel_adder (signal a,b:in std_logic_vector(3 downto 0);

signal cin: in std_logic;

signal s: out std_logic_vector(3 downto 0);

signal cout: out std_logic);

end digital_package;

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package body digital_package is

procedure full_adder(signal a,b,cin: in std_logic;

signal s,c: out std_logic) is

begin

s<=a xor b xor cin;

c<=(cin and(a xor b)) or (a and b);

end full_adder;

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procedure parallel_adder (signal a,b: in std_logic_vector(3 downto 0);

signal cin: in std_logic;

signal s: out std_logic_vector(3 downto 0);

signal cout: out std_logic) is

signal t:std_logic_vector(3 downto 0);

begin

full_adder(a(0),b(0),cin,s(0),t(0));

full_adder(a(1),b(1),t(0),s(1),t(1));

full_adder(a(2),b(2),t(1),s(2),t(2));

full_adder(a(3),b(3),t(2),s(3),cout);

end parallel_adder;

end digital_package;

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