   Returns the most common item in `list`, or `nil` if
   there isn't one.
   """
+  @spec most_common(list) :: any
   def most_common([n]), do: n
   def most_common(list) do
     list

    frequencies(%{high: 3, med: 3, low: 4, pairs: 1}) # 8
  end

  def generate_ai_set("easy") do
    frequencies(%{high: 3, med: 5, low: 4, pairs: 0}) # 12
  end

  @doc """
  Generates a list of letters based on the difficulty.
  """
  @spec generate_set(String.t) :: list
  def generate_set(difficulty) do
    letters = generate_player_set(difficulty)
  end

  @spec generate_player_set(String.t) :: list
  def generate_player_set("easy") do
    frequencies(%{high: 5, med: 4, low: 2, pairs: 2}) # 20
  end
