Show that the context$-$free languages are closed under subsequence. $($When $w=w_{1}cdots{w}_{n}in$

${}^{***},$subseq$(w)=\{\begin{array}{c}{w}_{i_1}cdots{w}_{i_l}\text{:}0ln\end{array}$ and $(L)=u_{\text{w}\text{s}\text{u}\text{b}\text{L}}\{$:$(w))(\{0^n{1}^n$:$n0\})=0^{***}{1}^{***}$