A user-defined function inputs several variable-length arrays.

It's task is to sum positive changes between all cells in some of its input
arrays;
while also summing negative changes in other input arrays.

Example one, sum positive changes only in cells A1:An (offset by n cells).
Ignore negative changes. And sum negative changes in cells B1:Bn (offset by
n cells). Ignore positive changes.

Example two, sum positive changes only in cells A50:A-n (offset by -n
cells). Ignore negative changes. And sum negative changes in cells B70:B-n
(offset by -n cells). Ignore positive changes. For clarity, this example
resolves to A50:A20 and B70:B10.

How would one best specify to the function that the second example requires
a start at A50 and B70 and descending; while the first example requires a
start at A1 and B1 and ascending?