def ensure_existence(f):
    """ Ensures that method is not marked as non_existent

    Parameters
    ----------
    f  Method

    Raises
    ------
    NotImplementedError if the method is marked as non existent

    Returns
    -------
    Method f
    """
    if getattr(f, 'not_existing', False):
        raise NotImplementedError('The distribution has no ' + f.__name__ + ' function. '
                                  'You may use an adapter that supports its approximation.')

    return f