parent(kim,holly).
parent(margaret,kim).
parent(margaret,kent).
parent(esther,margaret).
parent(herbert,margaret).
parent(herbert,jean).
greatgrandparent(GGP,GGC) :- 
  parent(GGP,GP), parent(GP,P), parent(P,GGC).


change(e,w).
change(w,e).

move([X,X,Goat,Cabbage],wolf,[Y,Y,Goat,Cabbage]) :-
  change(X,Y).
move([X,Wolf,X,Cabbage],goat,[Y,Wolf,Y,Cabbage]) :-
  change(X,Y).
move([X,Wolf,Goat,X],cabbage,[Y,Wolf,Goat,Y]) :-
  change(X,Y).
move([X,Wolf,Goat,C],nothing,[Y,Wolf,Goat,C]) :-
  change(X,Y).

oneEq(X,X,_).
oneEq(X,_,X).

safe([Man,Wolf,Goat,Cabbage]) :-
  oneEq(Man,Goat,Wolf),
  oneEq(Man,Goat,Cabbage).

solution([e,e,e,e],[]).
solution(Config,[Move|Rest]) :-
  move(Config,Move,NextConfig),
  safe(NextConfig),
  solution(NextConfig,Rest).