! A brief explanation of each line of trees:
! 1. Calculate how many spots we need to check for trees.
! Build a virtual sequence of this many slopes as pairs. For
! example, if our slope is { 1 3 } and we need to check 4
! spots, the virtual sequence looks like
! { { 1 3 } { 1 3 } { 1 3 } { 1 3 } }.
! 2. Take the cumulative sum of the virtual sequence, but mod
! the column by the width of the input matrix. This gives us
! a sequence of indices we need to check for trees in the
! input matrix.
! 3. Get the elements from the input matrix using the list of
! indices we built. Count how many of them are octothorps.