n>
  <span class="c">!!       |       |       |</span>
  <span class="c">!!     --+---v---+---v---+---v--</span>
  <span class="c">!!       |       |       |</span>
  <span class="c">!! j-1   u   f   u   f   u   f</span>
  <span class="c">!!       |  i-1  |   i   |  i+1</span>
  <span class="c">!!</span>
  <span class="c">!! The differentiation range is [2:size(f,dim=1)-1,2:size(f,dim=2)-1], so 1</span>
  <span class="c">!! halo cell on each end needs to be set priod to calling this function.</span>
  <span