*** 2
?- go_well_together(Y,rabbit).
returns yes because of floundering (this answer is unsound).
?- incompatible(Y,rabbit).
fails because, in turn,
?- likes(Y,rabbit).
and
?- likes(rabbit,Y).
succeed.

?- go_well_together(python,rabbit).
correctly reports failure.
Unsoundness of the previous answer is evident, as this goal is an instance of ?- go_well_together(Y,rabbit)., which succeeded, whereas this one fails.

*** 3
nonorphan(X) :- mother(_,X).
nonorphan(X) :- father(_,X).
orphan(X) :- person(X), \+ nonorphan(X).

*** 4
len([],0).
len([H|T],N):-len(T,M),N is M+1.

*** 5
lenp([],0).
lenp([H|T],s(N)):-lenp(T,N).

*** 6
add(X,0,X).
add(X,s(Y),s(Z)):-add(X,Y,Z).

*** 7
:- use_module(library(lists)).
stars :- length(L,10), member(_,L), write(*), fail; nl.

*** 8
table(L) :- member(X,L), member(Y,L), Z is X*Y, write(X*Y=Z), nl, fail; nl.