public function testMap()
{
$addTwo = function ($x) {
return $x + 2;
};
$id = function ($x) {
return $x;
};
$a = new Right(5);
$b = new Left(4);
$this->assertEquals($a->map($addTwo)->either($id, $id), 7, 'Maps a Right.');
$this->assertEquals($b->map($addTwo)->either($id, $id), 4, 'Maps a Left.');
}