?- [user].
|: person(gertie).
|: person(herbert).
|: person(miriam).
|: person(wallace).
|: exchange(al).
|: exchange(be).
|: exchange(pe).
|: exchange(sl).
|: year(1928).
|: year(1929).
|: year(1932).
|: year(1935).
|:
|: not_first_exchange(A, B, C) :- first_exchange(A, \+ B, C).
|: first_exchange(wallace, al, _).
|: first_exchange(_, pe, 1929).
|: not_first_exchange(_, al, 1932).
|: not_first_exchange(_, be, 1932).
|: not_first_exchange(_, be, 1935).
|: not_first_exchange(gertie, sl, _).
|: not_first_exchange(herbert, sl, _).
|: not_first_exchange(herbert, be, _).
^D
?- first_exchange(A, B, C), person(A), exchange(B), year(C).
A = wallace,
B = al,
C = 1928 ;
A = wallace,
B = al,
C = 1929 ;
A = wallace,
B = al,
C = 1932 ;
A = wallace,
B = al,
C = 1935 ;
A = gertie,
B = pe,
C = 1929 ;
A = herbert,
B = pe,
C = 1929 ;
A = miriam,
B = pe,
C = 1929 ;
A = wallace,
B = pe,
C = 1929.