Description:
            Bottom k breaking
        Parameters:
            k: the number of alternatives to break from lowest rank

def _bot(self, k):
        """
        Description:
            Bottom k breaking
        Parameters:
            k: the number of alternatives to break from lowest rank
        """
        if k < 2:
            raise ValueError("k smaller than 2")
        G = np.ones((self.m, self.m))
        np.fill_diagonal(G, 0)
        for i in range(self.m):
            for j in range(self.m):
                if i == j:
                    continue
                if i <= k and j <= k:
                    G[i][j] = 0
        return G