You mean that thesize of x is 10x10000, right? Not that x is the vector [10,10000], which is what you wrote.
There is no unique inverse to the sum() command. Think about it. If I tell you that 2+2 equals 4, there is no way to for you to undo that, and obtain 2 and 2 from 4. It could have been 1 and 3, or 0 and 4, or -1.1 and 5.1, etc.
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