Add the following expression to LET language:

Expression ::= unpack {Identifier} = Expression in Expression

so that unpack x y z = lst in ... binds x ,y and z to the elements of lst if lst is a list of exactly three elements, and reports an error otherwise. For example, the value of

let u = 7

in unpack x y = cons(u,cons(3,emptylist))

in - (x , y)

should be 4.