class SetTerminator {
	
	public function executeFilter (Set $set) {
		$set->filter(function ($value) {
			return false;
		});
	}
	
	public function executePop (Set $set) {
		while (!$set->isEmpty()) {
			$set->pop();
		}
	}
	
}

class Set implements ArrayAccess, Iterator, Countable {
	
	private $data;
	private $position = 0;
	
	public function __construct (array $data = []) {
		$this->data = [];
	}
	
	public function pop () {
		return array_pop($this->data);
	}
	
	public function filter (Closure $callback) {
		$this->data = array_filter($this->data, $callback);
	}
	
	public function isEmpty () {
		return empty($this->data);
	}
	
	// ArrayAccess
	public function offsetGet ($offset) {
		return $this->offsetExists($offset) ? $this->data[$offset] : false;
	}
	
	public function offsetExists ($offset) {
		return isset($this->data[$offset]);
	}
	
	public function offsetSet ($offset, $value) {
		return $this->data[$offset] = $value;
	}
	
	public function offsetUnset ($offset) {
		unset($this->data[$offset]);
	}
	
	// Iterator
	public function rewind () {
        $this->position = 0;
    }

    public function current () {
        return $this->items[$this->position];
    }

    public function key () {
        return $this->position;
    }

    public function next () {
        $this->position ++;
    }

    public function valid () {
        return isset($this->items[$this->position]);
    }
    
    // Countable
    public function count () {
    	return count($this->data);
    }
	
}

$set_one = new Set(range(1, 10));
$set_two = new Set(range(1, 20));

$terminator = new SetTerminator;
$terminator->executeFilter($set_one);
$terminator->executePop($set_two);

var_dump(count($set_one), count($set_two)); // int(0), int(0)