Finding the rank of a matrix based on a variable

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How can I find the rank of the matrix $B_{a} = \begin{bmatrix} -1 & -1 & 0 & 1\\ 1 & 1 & 0 & -1\\ a & 1 & 1 & 0\\ 0 & 1 & a & 1 \end{bmatrix}$ based on the possible values of $a$?