Chapter 2: Getting Started

@laurocaetano, @rmascarenhas, @andrielfn and @rcillo After a lot of effort on this chapter I tried my best to solve exercises and problems, but couldn't finish all of them.
But maybe I think it should have some more thoughts on what the *invariant* is. Something by the lines:

Before each loop, the subarray `C[1 .. i -1]` contains the sum of the subarrays `A[1 .. i - 1]` and `B[1.. i - 1]`. And the value of `carry` is the value of the xpto.

Then you chould check this invariant in the initialization condition (before the first loop run), then the maintenance and termination as well.