public function addRecurrenceType($type, $recurrenceType)
	{
		if (is_string($recurrenceType) and !class_exists($recurrenceType)) {
			throw new \InvalidArgumentException("Class {$recurrenceType} does not exist.");
		} elseif (!in_array('Plummer\Calendarful\Recurrence\RecurrenceInterface', class_implements($recurrenceType))) {
			throw new \InvalidArgumentException('File or File path required.');
		}

		$this->recurrenceTypes[$type] = is_string($recurrenceType) ?
			$recurrenceType :
			get_class($recurrenceType);
	}