public function testFoldLeftRight()
{
$seq = new Sequence(['a', 'b', 'c']);
$rsLeft = $seq->foldLeft('', function ($a, $b) {
return $a . $b;
});
$rsRight = $seq->foldRight('', function ($a, $b) {
return $a . $b;
});
$this->assertEquals('abc', $rsLeft);
$this->assertEquals('abc', $rsRight);
}