-- 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 pathfinder (world : rows=>cols=>Int) : cols=>Int =
  yield_state zero $ \costsRef.
    for r.
      costs = get costsRef
      costsRef := for c. world.r.c + (min costs.c $ (min costs.(c -| 1)
                                                         costs.(c +| 1)))