public function testFilter()
{
$array = ["a" => 1, "b" => 2, "c" => 3];
$map = new Map($array);
$filtered = $map->filter(function ($_) {
return $_ & 1;
});
$this->assertTrue($filtered instanceof Map);
$this->assertEquals($array, $map->toArray());
//check original Map is not altered
$this->assertEquals(2, $filtered->count());
$this->assertEquals(1, $filtered->get("a"));
$this->assertEquals(3, $filtered->get("c"));
}