def concatenate_residue_labels(labels):
    """
    Concatenate residue labels.

    This function is a generator.

    Parameters
    ----------
    labels : numpy array of shape (N, M)
        Where N is the number of rows, and M the number of columns
        with the labels to be concatenated.
    """
    empty_join = ''.join
    return (empty_join(res_label) for res_label in labels)