Question 1:

Prove that the definition of APPEND is associative. In other words,
APPEND(x,APPEND(y,z)) is equivalent to APPEND(APPEND(x,y),z). [Hint:  use
induction on the length of the list bound to x.]

Question 2:

Show how the FLAT function can be transformed to yield the
FLAT2 function using the transformations we described for adding an
accumulator variable and thereby making the function be tail recursive.