In the following code using the list abstract data type, A and B are constants.

function prepend$($e$,$ lst$)\{$ return lst $=$ node$($e$,$ lst$)$; $\}$

const sum $=$ lst $=>$ lst$.$isEmpty$()?0$ : lst$.$head$()+$ sum$($lst$.$tail$())$;

let lst$1=$ node$($A$,$ node$($B$,$ empty$()))$;

prepend$($A$,$ lst$1)$;

let lst$2=$ prepend$($B$,$ lst$1)$;

console.log$($sum$($lst$1))$;

console.log$($sum$($lst$2))$;

If the two numbers printed are $8$ and $4,$ what is the value of A$?$