function A = trizero(A, upper)
  [m n] = size(A);
  if (m ~= n)
    A = [];
    return;
  end

  if (upper)
    for i = 1:n-1
      for j = i+1:n
        A(i,j) = 0;
      end
    end
  else
    for i = 2:n
      for j = 1:i-1
        A(i,j) = 0;
      end
    end
  end
end