I'm reviewing an algorithm and it all makes sense except for this line:
$$ \boldsymbol{b} \leftarrow (||\boldsymbol{b}|| \_{2} - \beta)_{+} \widehat{\boldsymbol{b}} $$
$\boldsymbol b$ is an $N\times 1$ vector and $\beta$ is a constant. I'm not sure what the subscript plus denotes.