Create filter classes

abstract class filterset {

  public function __construct(filter[], int $jointype): void;

  public function add_filter(string $name, filter $filter): self;

  public function validate(): bool {

    // Ensure that only specified filter types are here.

    // Use a new function to get the description of those - e.g. $this->get_valid_filtertypes();

  }

}

abstract class filter {

  const JOINTYPE_NONE = 0;

  const JOINTYPE_ANY = 1;

  const JOINTYPE_ALL = 2;

  public function __construct(int $jointype, array $values): void;

  public function add_value($value): self;

}

class group extends filter {

  public function add_value(int $value): self {

    $this->values[] = $value;

  }

  public function validate(?int $courseid): bool {

    // Check that they're all valid groups in the (optionally-specified) course.

  }

}