def.exs

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

def next_state do
  ("o", live_count) when live_count in 2..3 -> "o"
  ("o", _) -> "."
  (".", live_count) when live_count === 3 -> "o" 
  (".", _) -> "."
end