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function retur = trizero(A, upper)
    if size(A, 1) ~= size(A, 2)
        retur = [];
        return;
    end
    N = size(A, 1);
    for i = 1:N-1
        if upper
            A(i, i+1:N) = 0;
        else
            A(i+1:N, i) = 0;
        end
    end
    retur = A;
end