% likeable.pl

likes(eve, pie).
likes(al,  eve).
likes(eve, tom).
likes(eve, eve).

food(apple).
food(pie).

person(tom).

liked(V) :- likes(W,V).

likeable(V) :- likes(W,V), person(V).

/* Example run:

   % prolog
   SICStus 3.7: Tue Sep 08 19:55:50 MET DST 1998
   Licensed to cs.rutgers.edu
   | ?- [likeable].
   {consulting likeable.pl...}
   {Warning: [W] - singleton variables in liked/1 in lines 10-12}
   {Warning: [W] - singleton variables in likeable/1 in lines 12-14}
   {likeable.pl consulted, 0 msec 1992 bytes}

   yes
   | ?- likeable(A).

   A = tom ? ;

   no
   | ?- trace.
   {The debugger will first creep -- showing everything (trace)}

   yes
   {trace}
   | ?- likeable(A).
	   1      1 Call: likeable(_246) ? 
	   2      2 Call: likes(_740,_246) ? 
   ?       2      2 Exit: likes(eve,pie) ? 
	   3      2 Call: person(pie) ? 
	   3      2 Fail: person(pie) ? 
	   2      2 Redo: likes(eve,pie) ? 
   ?       2      2 Exit: likes(al,eve) ? 
	   3      2 Call: person(eve) ? 
	   3      2 Fail: person(eve) ? 
	   2      2 Redo: likes(al,eve) ? 
   ?       2      2 Exit: likes(eve,tom) ? 
	   3      2 Call: person(tom) ? 
	   3      2 Exit: person(tom) ? 
   ?       1      1 Exit: likeable(tom) ? 

   A = tom ? ;
	   1      1 Redo: likeable(tom) ? 
	   2      2 Redo: likes(eve,tom) ? 
	   2      2 Exit: likes(eve,eve) ? 
	   3      2 Call: person(eve) ? 
	   3      2 Fail: person(eve) ? 
	   1      1 Fail: likeable(_246) ? 

   no
   {trace}
   | ?- halt.

*/