Sum the first n elements of arr recursively – the same problem as this module’s very first “Array Sum”
exercise, this time without a loop.
The recursive step reuses two ideas from earlier in this module at once: arr[0] peels off the first element (an
ordinary array index), and arr + 1 – pointer arithmetic – produces a pointer to “the array starting one
element later,” which paired with n - 1 describes exactly the remaining elements. array_sum(arr + 1, n - 1) is
a completely ordinary recursive call on a smaller version of the same problem; it just happens that “smaller” here
means “starting one element further into the same block of memory,” not a smaller number passed by value.