-- Assumes that off >= 0
def (+|) (i:n) (off:Int) : n =
  newOrd = ordinal i + off
  case newOrd < size n of
    True  -> unsafe_from_ordinal _ newOrd
    False -> i

-- Assumes that off >= 0
def (-|) (i:n) (off:Int) : n =
  newOrd = ordinal i - off
  case 0 <= newOrd of
    True  -> unsafe_from_ordinal _ newOrd
    False -> i

def stencil (input : nx=>ny=>nz=>Float) : nx=>ny=>nz=>Float =
  c0 = 1.0 / 6.0
  c1 = c0 * c0
  (xs, ys, zs) = (size nx, size ny, size nz)
  for x y z.
    (xi, yi, zi) = (ordinal x, ordinal y, ordinal z)
    case xi == 0 || xi == (xs-1) || yi == 0 || yi == (ys-1) || zi == 0 || zi == (zs-1) of
      True  -> input.x.y.z
      False ->
        neigh = (input.x.y.(z -| 1) + input.x.y.(z +| 1) +
                 input.x.(y -| 1).z + input.x.(y +| 1).z +
                 input.(x -| 1).y.z + input.(x +| 1).y.z)
        input.x.y.z * c0 + neigh * c1