Simple Signed Adder is complex to understand

Hi guys, still asking beginner questions, sorry. I have this code and cant figure out what the '0' in ('0' & cin) is doing. This is what I think:

Lets say we we need to do add two to negative four:

(2)0010 + (-4)1101 where the answer is (-2)11110 (last bit '1' indicates the negative sign) so,

(a_sig(n-1) & a_sig) 00010

+ (b_sig(n-1) & b_sig) 11101

+ ('0' & cin) 00

ans 11111 (which is incorrect)

What am I getting wrong?http://www.alteraforum.com/forum//images/icons/icon8.png

Also is this signed adder actually efficient cause lots of examples suggest loads of other ways to do it and I can't figure out which one to stick to or it depends on the application?

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity examples is

generic(n : integer := 4); -- number of input bits

port(a, b: in std_logic_vector(n-1 downto 0);

cin: in std_LOGIC := '0';

sum: out std_LOGIC_VECTOR(n downto 0));

end examples;

architecture add_sub of examples is

signal a_sig, b_sig: signed(n-1 downto 0);

signal sum_sig: signed(n downto 0);

begin

--convert to signed

a_sig <= signed(a);

b_sig <= signed(b);

--add

sum_sig <= (a_sig(n-1) & a_sig) + (b_sig(n-1) & b_sig) + ('0' & cin);

sum <= std_logic_vector(sum_sig);

end add_sub;