def.exs

def next_state
  (board, x, y) ->
    cell = cell_at(board, x, y)
    live_count = live_neighbors(board, x, y)
    next_state(cell, live_count)
  end
  ("o", live_count) when live_count in 2..3 -> "o"
  ("o", _) -> "."
  (".", live_count) when live_count === 3 -> "o" 
  (".", _) -> "."
  (_, x, y) when (x < 0 or y < 0) ->  "."
 

# This would internally define both next_state/3 and next_state/2
# Since these are distinctions that the user largely doesn't care about