 
   The returned value of the environment variable
   `varname` is a string, or `nil` if the environment
-  variable is undefined.
+  variable is not set.
 
   ## Examples
 
       iex> System.get_env("PORT")
       "4000"
 
-      iex> System.get_env("NOT_DEFINED")
+      iex> System.get_env("NOT_SET")
       nil
 
   """

  @doc """
  Returns the value of the given environment variable or `:error` if not found.

  If `varname` is a defined environment variable, then `{:ok, value}` is returned
  where `value` is a string. If `varname` is not defined, `:error` is returned.

  ## Examples

      iex> System.fetch_env("PORT")
      {:ok, "4000"}

      iex> System.fetch_env("NOT_DEFINED")
      :error

  """

  Returns the value of the given environment variable or raises if not found.

  Same as `get_env/1` but raises instead of returning `nil` when the variable is
  not set.

  ## Examples

      iex> System.fetch_env!("PORT")
      "4000"

      iex> System.fetch_env!("NOT_SET")
      ** (ArgumentError) could not fetch environment variable "NOT_SET" because it is not set

  """
  @doc since: "1.9.0"
