Question 3

simplify1(end):-!.
simplify1(Exp+X):-
    (  (retract(v(X,N)),!,  %% I put the retract in the "or" and changed the sides of the "or"
        N1 is N+1,
        assert(v(X,N1)))
    ;
        assert(v(X,1))
    ),
    simplify1(Exp).
simplify1(X):-
    atomic(X),  %% I add the atomic check
    simplify1(end+X).


list_to_output([C/N|List],Sum,Out):-
    list_to_output(List,Sum1,Out1), %% I changed this line place
    (   atom(C),
        N>1,!,
        Out=Out1+C*N,
        Sum = Sum1 %% I added it
    ;   atom(C),!,
        Out=Out1+C,
        Sum = Sum1 %% I added it
    ;   integer(C),
        Sum is Sum1+(C*N),
        Out = Out1 % I added it
    ).
list_to_output([],0,'').

simplify(Expression,Out1):-
    simplify1(Expression),
    findall(C/N,v(C,N),List),
    list_to_output(List,Sum,Out),
    Out1 =Out+Sum.