remove vestigial prolog/ directory (#444)

src/examples/expert_system.pl

@@ -0,0 +1,42 @@
+:- use_module(library(dcgs)).
+:- use_module(library(reif)).
+
+animals([animal(dog, [is_true('has fur'), is_true('says woof')]),
+         animal(cat, [is_true('has fur'), is_true('says meow')]),
+         animal(duck, [is_true('has feathers'), is_true('says quack')])]).
+
+animal(A) :-
+    animals(Animals),
+    Known0 = [],
+    phrase(any_animal(Animals, A), [Known0], _).
+
+any_animal([Animal|Animals], A) -->
+    any_animal_(Animal, Animals, A).
+
+any_animal_(animal(A0, []), Animals, A) -->
+    (   { A0 = A }
+    ;   any_animal(Animals, A)
+    ).
+any_animal_(animal(A0, [C|Cs]), Animals, A) -->
+    state0_state(Known0, Known),
+    { condition_truth(C, T, Known0, Known) },
+    next_animal(T, animal(A0,Cs), Animals, A).
+
+next_animal(yes, Animal, Animals, A)  --> any_animal([Animal|Animals], A).
+next_animal(no, _, Animals, A)        --> any_animal(Animals, A).
+
+state0_state(S0, S), [S] --> [S0].
+
+condition_truth(is_true(Q), Answer, Known0, Known) :-
+    if_(known_(Q,Answer,Known0),
+        Known0 = Known,
+        ( writeq([Q, ?]), nl,
+	  read(Answer),
+          Known = [known(Q,Answer)|Known0])).
+
+known_(What, Answer, Known, Truth) :-
+    if_(memberd_t(known(What,yes), Known),
+        ( Answer = yes, Truth = true ),
+        if_(memberd_t(known(What,no), Known),
+            ( Answer = no, Truth = true),
+            Truth = false)).