function _identity(x) {
    return x;
  }
Returns the first element of the given list or string. In some libraries
this function is named `first`.

@func
@memberOf R
@since v0.1.0
@category List
@sig [a] -> a | Undefined
@sig String -> String | Undefined
@param {Array|String} list
@return {*}
@see R.tail, R.init, R.last
@example

     R.head(['fi', 'fo', 'fum']); //=> 'fi'
     R.head([]); //=> undefined

     R.head('abc'); //=> 'a'
     R.head(''); //=> undefined