public function test_cache() {
    $mock = $this->getMockBuilder( 'Plural_Forms' )
      ->setMethods( array( 'execute' ) )
      ->setConstructorArgs( array( 'n != 1' ) )
      ->getMock();

    $mock->expects( $this->once() )
      ->method( 'execute' )
      ->with( $this->identicalTo( 2 ) )
      ->will( $this->returnValue( 1 ) );

    $first  = $mock->get( 2 );
    $second = $mock->get( 2 );
    $this->assertSame( $first, $second );
  }