public function testItCanCalculatePerimeter()
{
$circle = new Circle(0);
$this->assertEquals(0, $circle->perimeter());
$circle = new Circle(2);
$value = 2 * pi() * 2;
$this->assertEquals($value, $circle->perimeter());
$circle = new Circle(3.14);
$value = 2 * pi() * 3.14;
$this->assertEquals($value, $circle->perimeter());
}
