def remove_symbol_from_dist(dist, index):
    '''
    prob is a ndarray representing a probability distribution.
    index is a number between 0 and and the number of symbols ( len(prob)-1 )
    return the probability distribution if the element at 'index' was no longer available
    '''
    if type(dist) is not Distribution:
        raise TypeError("remove_symbol_from_dist got an object ot type {0}".format(type(dist)))

    new_prob = dist.prob.copy()
    new_prob[index]=0
    new_prob /= sum(new_prob)
    return Distribution(new_prob)