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Three extra bits should duplicate three of the four main bits. If the duplicates are the same, then the non-duplicated bit is corrupted; otherwise, we look for where the difference lies.
Added 3 minutes later
Ah, there they might not corrupt anything at all...
Okay, let's say we received the message 0011011. How can we tell which bit is corrupted?
Added 1 minute later
Ah, there they might not corrupt anything at all...
Всё не так плохо как Вы думаете. Всё намного хуже!