Description:
            Top k breaking
        Parameters:
            k: the number of alternatives to break from highest rank

def _top(self, k):
        """
        Description:
            Top k breaking
        Parameters:
            k: the number of alternatives to break from highest rank
        """
        if k > self.m:
            raise ValueError("k larger than the number of alternatives")
        G = np.ones((self.m, self.m))
        #np.fill_diagonal(G, 0)  # erroneous code from prefpy
        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