Consider the grammar below.

$($:$ss$:$)-($:$s$:$)|($:$s$:$)$;$($:$ss$:$)|$

$($:$s$:$)-($:$v$:$)=($:$e$:$)$

$($:$e$:$)-($:$e$:$)+($:$t$:$)|($:$e$:$)-($:$t$:$)|($:$t$:$)$

$($:$t$:$)$ const

$($:$v$:$)a|b|c|d|$

In the rightmost derivation of $"a=$const;$b=a",$ what is the $5$ th sentential form? $($the first one is $($:$s$:$)$;$($:$v$:$)=($:$t$:$)$