Upper bounds on|C_2|for a uniquely decodable code pair(C_1, C_2)for a two-access binary adder channel

An algebraic and a combinatorial upper bound are derived on
|C_{2}|
given the code
C_{1}
, where
(C_{l}, C_{2})
is a uniquely decodable code pair
(C_{1}, C_{2})
for a two-access binary adder channel. A uniquely decodable code with rate pair
(0.5170, 0.7814)
is also described.