\begin{verbatim}
architecture Behavioral of Add1BitB is

begin

Adder: process(X, Y, Ci) is variable buf: STD_LOGIC_VECTOR(2 downto 0);
begin
  buf(0) := X;
  buf(1) := Y;
  buf(2) := Ci;

  case (buf) is
    when "000" =>
      O  <= '0';
      Co <= '0';
    when "001" =>
      O  <= '1';
      Co <= '0';
    when "010" =>
      O  <= '1';
      Co <= '0';
    when "011" =>
      O  <= '0';
      Co <= '1';
    when "100" =>
      O  <= '1';
      Co <= '0';
    when "101" =>
      O  <= '0';
      Co <= '1';
    when "110" =>
      O  <= '0';
      Co <= '1';
    when "111" =>
      O  <= '1';
      Co <= '1';
    when others =>
      O  <= '0';
      Co <= '0';
  end case;

end process Adder;

end Behavioral;
\end{verbatim}