
  [;1m-spec hd(List) -> term() when List :: [term(), ...].[0m

  Returns the head of [;;4mList[0m, that is, the first element, for
  example:

    > hd([1,2,3,4,5]).
    1

  Allowed in guard tests.

  Failure: [;;4mbadarg[0m if [;;4mList[0m is the empty list [;;4m[][0m.
