how that the context$-$free languages are closed under subsequence. $($When w $=$ w$1$ wn in

$\backslash$Sigma $,$ subseq$($w$)=\{$wi$1$ wi$$ : $0<=<=$ n and $1<=$ i$1<$ i$2<<$ i$<=$ n$\}$; and subseq$($L$)=$S

w in L subseq$($w$)).$ For example, subseq$(\{0$n$1$n : n $>=0\})=01.$