def __init__(self, tier=None, n=1, with_radius=0, with_alt=False):
    """Create a new TierStats instance.

    :param tier: (either sppasTier or list of them)
    :param n: (int) n-gram value
    :param with_radius: (int) 0 to use Midpoint, negative value
    to use R-, positive value to use R+
    :param with_alt: (bool) Use or not use of alternative labels

    """
    self.tier = tier
    self._with_radius = with_radius
    self._with_alt = with_alt
    self._ngram = 1
    self.set_ngram(n)

def get_ngram(self):
    """Returns the n-gram value."""
    return self._ngram

def get_with_radius(self):
    """Returns how to use the radius in duration estimations.

        0 means to use Midpoint, negative value means to use R-, and 
        positive value means to use R+.

        """
    return self._with_radius

def get_with_alt(self):
    """Return if alternative labels will be used or not."""
    return self._with_alt

def get_tier(self):
    """Return the tier to estimate stats."""
    return self.tier

def set_with_radius(self, with_radius):
    """Set the with_radius option, used to estimate the duration.

        :param with_radius: (int) Fix the with_radius option

        with_radius can take the following values:

            - 0 means to use midpoint;
            - negative value means to use (midpoint-radius);
            - positive radius means to use (midpoint+radius).

        """
    self._with_radius = int(with_radius)

def set_withalt(self, withalt):
    """Set the withalt option, used to select the labels.


            - False means to use only the label with the higher score
            of each annotation
            - True means to use all labels of each annotation

        """
    self._with_alt = withalt

def set_ngram(self, n):
    """Set the n value of the n-grams.

        It is used to fix the history size (at least =1).

        """
    n = int(n)
    if 0 < n <= MAX_NGRAM:
        self._ngram = n
    else:
        raise InsideIntervalError(n, 1, MAX_NGRAM)

def ds(self):
    """Create a DescriptiveStatistic object for the given tier. 

        :returns: (DescriptiveStatistic)

        """
    ltup = self.__tiers_to_tuple()
    ngrams = list()
    for t in ltup:
        ngrams.extend(self.__ngrams(t))
    items = sppasTierStats.tuple_to_dict(ngrams)
    return sppasDescriptiveStatistics(items)

@staticmethod
def tuple_to_dict(items):
    """Convert into a dictionary.

        :param items: (tuple) the ngram items
        :returns: dictionary key=text, value=list of durations.

        """
    d = dict()
    for item in items:
        dur = sum([i[1] for i in item])
        text = ' '.join([i[0] for i in item])
        if text not in d:
            d[text] = []
        d[text].append(dur)
    return d

def __tiers_to_tuple(self):
    """Return a list of tuples of label/duration pairs."""
    tiers = self.tier
    if not isinstance(self.tier, list):
        tiers = [self.tier]
    return [self.__tier_to_tuple(tier) for tier in tiers]

def __tier_to_tuple(self, tier):
    """Return a tuple of label/duration pairs for a given tier.

        :param tier: (Tier)
        :returns: tuple

        """
    l = list()
    for ann in tier:
        labels = ann.get_labels()
        if labels is None:
            continue
        if len(labels) == 0:
            continue
        contents = list()
        for label in labels:
            if self._with_alt is False:
                contents.append(label.get_best())
            else:
                for tag, score in label:
                    if tag is not None:
                        contents.append(tag)
        dur = 0.0
        if len(labels) == 1:
            duration = ann.get_location().get_best().duration()
            dur = duration.get_value()
            if self._with_radius < 0:
                dur += duration.get_margin()
            elif self._with_radius > 0:
                dur -= duration.get_margin()
        for tag in contents:
            l.append((tag.get_content(), dur))
    return tuple(l)

def __ngrams(self, items):
    """Yield a sequences of ngrams. 

        :param items: (tuple) the ngram items

        """
    l = list()
    size = len(items)
    if size - self._ngram > 0:
        limit = size - self._ngram + 1
        for i in range(limit):
            l.append(items[i:i + self._ngram])
    return l