purchase(Product) :- 
	product_available(Product),
	sufficient_payment(Product, Change),
	dispense_product(Product),
	return_change(Change), !.
purchase(Product) :-
	product_available(Product),
	product_cost(Product, Cost),
	print(Product, ' costs ', Cost, ' cents'), !.
purchase(Product) :-
	print(Product, ' is not available'),!.
insert_coin(Coin) :-
	value(Coin,Worth),
	Worth > 0,
	coin_accept(Worth), !.
insert_coin(Coin) :-
	print('coin reject'), !.
coin_return :-
	deposit(X),
	change(X),
	clear_deposit,
	print(X, ' cents returned'), 
	!.
coin_return :-
	print('No coins deposited').
product_available(Product) :-
	quantity(Product, Qty),
	Qty > 0, !.
sufficient_payment(Product, Change) :-
	product_cost(Product, Cost),
	deposit(Amount),
	Amount >= Cost,
	Change is Amount - Cost, !.	
sufficient_payment(Product, _) :-
	product_cost(Product, Cost),
	deposit(Amount),
	Deficit is Cost - Amount,
	print('Deposit additional ', Deficit, ' cents'),
	fail, !.
dispense_product(Product) :- 
	print(Product, ' dispensed'),
	quantity(Product,Qty),
	retract(quantity(Product, Qty)),
	Newqty is Qty - 1,
	assert(quantity(Product,Newqty)), !.
return_change(Change) :- 
	Change > 0,
	change(Change), 
	print(Change, ' cents change returned'), 
	clear_deposit, !.
return_change(0) :- print('No change').
change(Change) :- change_half_dollar(Change,X).
change_half_dollar(Change, X) :-
	Change >= 50,
	X is Change - 50,
	print('dispense half_dollar'),
	print('rem: ',X),!,
	change_half_dollar(X, Y).
change_half_dollar(X, Y) :-
	change_quarter(X,Y).
change_quarter(Change, X) :-
	Change >= 25,
	X is Change - 25,
	print('dispense quarter'),
	print('rem: ',X),!,
	change_quarter(X, Y).
change_quarter(X, Y) :-
	change_dime(X,Y).
change_dime(Change, X) :-
	Change >= 10,
	X is Change - 10,
	print('dispense dime'),
	print('rem: ',X), !,
	change_dime(X, Y).
change_dime(X,Y) :-
	change_nickel(X, Y).
change_nickel(Change, X) :-
	Change >= 5,
	X is Change - 5,
	print('dispense nickel'),
	print('rem: ',X),!,
	change_nickel(X, Y).
change_nickel(X,Y) :-
	print('change OK'), !.
clear_deposit :- retract(deposit(X)).
coin_accept(X) :- 
	deposit(Y), 
	Z is Y + X,
	retract(deposit(Y)),
	assert(deposit(Z)),
	print('Total deposit ', Z, ' cents'), !.
coin_accept(X) :-
	assert(deposit(X)),
	print('Total deposit ', X, ' cents'), !.
value(nickel, 5).
value(dime, 10).
value(quarter, 25).
value(half-dollar, 50).
value(slug, 0).
quantity(pretzels, 5).
quantity(chips, 5).
quantity(cookies, 5).
quantity(doughnuts, 5).
product_cost(pretzels, 50).
product_cost(chips, 45).
product_cost(cookies, 55).
product_cost(doughnuts, 60).