! 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.