设 $A=\left(\begin{array}{l}1 \\ 0 \\ 2\end{array}\right)\left(\begin{array}{lll}1 & -1 & 0\end{array}\right), B=\left(\begin{array}{ccc}1 & -1 & 2 \\ 2 & a & 1 \\ -1 & 3 & 0\end{array}\right)$ ，若 $R(A B+B)=2$ ， 求 $a$.
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