def _fit_one_ova(tr_class_idx, multi_ova, dataset, verbose):
    """Fit a OVA classifier.

    Parameters
    ----------
    tr_class_idx : int
        Index of the label against which the classifier should be trained.
    multi_ova : CClassifierMulticlassOVA
        Instance of the multiclass OVA classifier.
    dataset : CDataset
        Training set. Must be a :class:`.CDataset` instance with
        patterns data and corresponding labels.
    verbose : int
        Verbosity level of the logger.

    """
    multi_ova.verbose = verbose
    multi_ova.logger.info('Training against class: {:}'.format(tr_class_idx))
    train_ds = multi_ova.binarize_dataset(tr_class_idx, dataset)
    classifier_instance = multi_ova._binary_classifiers[tr_class_idx]
    classifier_instance.verbose = multi_ova.verbose
    classifier_instance.fit(train_ds)
    return classifier_instance
