21. A method comprising: accessing, without initially decoding, an arithmetically coded bitstream that includes at least a first coded portion and a second coded portion, wherein the first coded portion and the second coded portion have been coded with a code having memory, such that decoding the second coded portion relies on an output from decoding the first coded portion; and replacing the first coded portion with a replacement arithmetically coded portion to produce a modified coded bitstream for which a decoding of the second coded portion will produce a common result based on either an output from decoding the first coded portion or an output from decoding the replacement arithmetically coded portion, wherein the replacement arithmetically coded portion generates a Context-based Adaptive Binary Arithmetic Coding (CABAC) state value that is the same as a CABAC state value generated by the first coded portion.
21. A method comprising: accessing, without initially decoding, an arithmetically coded bitstream that includes at least a first coded portion and a second coded portion, wherein the first coded portion and the second coded portion have been coded with a code having memory, such that decoding the second coded portion relies on an output from decoding the first coded portion; and replacing the first coded portion with a replacement arithmetically coded portion to produce a modified coded bitstream for which a decoding of the second coded portion will produce a common result based on either an output from decoding the first coded portion or an output from decoding the replacement arithmetically coded portion, wherein the replacement arithmetically coded portion generates a Context-based Adaptive Binary Arithmetic Coding (CABAC) state value that is the same as a CABAC state value generated by the first coded portion. 29. The method of claim 21 wherein the arithmetically coded bitstream is a CABAC coded bitstream.