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COPE the first practical network coding scheme which is developped on click

quite a bit of reorganisation between this version and previous.
-- encoded header now includes source IP addresses, which are also used as keys
-- TransmittedQueue is splitted into Decoder and SentStore
-- in SentStore, there's only one-level hashmap, key = "source ip" + "ip seq" (string concatenation)
-- PrintXor modified according to the header
-- ScrambleQueue also...