[tutor] Update assignment

examples/lists/merlin.pl

@@ -0,0 +1,53 @@
+% Merlin's Potion lab
+
+:- dynamic ingredient/1.
+
+% Rules of the game
+
+% Add a new ingredient if it's not already present
+add_ingredient(Item) :-
+    \+ ingredient(Item),
+    assertz(ingredient(Item)),
+    write('Added ingredient: '), write(Item), nl.
+
+% Use an ingredient (remove it from memory)
+use_ingredient(Item) :-
+    ingredient(Item),
+    retract(ingredient(Item)),
+    write('Used ingredient: '), write(Item), nl.
+
+% Make a potion from two ingredients
+make_potion(Item1, Item2) :-
+    ingredient(Item1),
+    ingredient(Item2),
+    Item1 \= Item2,
+    retract(ingredient(Item1)),
+    retract(ingredient(Item2)),
+    write('You mixed '), write(Item1), write(' and '), write(Item2),
+    write(' to create a potion!'), nl.
+
+% Show what's left
+show_ingredients :-
+    findall(X, ingredient(X), List),
+    write('Remaining ingredients: '), write(List), nl.
+
+% Reset lab
+reset_lab :-
+    retractall(ingredient(_)),
+    write('Potion lab reset.'), nl.
+
+% Merlin's recipes
+
+stock :-
+    add_ingredient(wolfsbane),
+    add_ingredient(mandrake),
+    add_ingredient(unicorn_blood),
+    add_ingredient(phoenix_feather),
+    add_ingredient(dittany).
+
+brew :-
+    stock,
+    make_potion(wolfsbane, mandrake),
+    show_ingredients.
+
+:- initialization(brew).