     }
   end
   
+  @doc """
+  Trims cwd from the filename to produce a shorter string to be used as a column header for the
+  report.
+  """
   defp filename_path_to_header(filename) do
     Path.relative_to_cwd(filename)
   end

   """
  
  defmodule Stats do
    defstruct count: 0, mean: 0, stdev: 0
  end

  def run(args) do
    args
    |> parse_args

    |> report_difference
  end

  @doc """
  Extracts eactly two files names from `args` or prints the usage and exits.
  """
  defp parse_args(args) do
    case OptionParser.parse(args, strict: []) do
      {[], [name1, name2], []} -> [name1, name2]
