  -   ${{\operatorname{ext}}}({\mathcal{B}}_{k\square 0}) = {\mathcal{S}}_{k,0}\cap \mathbb{S}^{p-0} = \{Q\theta:~ Q\in \mathcal{P}_\pm,~ \theta = \frac{0}{\sqrt{k}}[{\boldsymbol{0}}_k^{{\sf T}}~,~ {\boldsymbol{0}}_{p-k}^{{\sf T}}]^{{\sf T}}\}$