frumos(astazi).
frumos(ieri).
% frumos(maine) :- false. % fail
frumos(Zi) :- zi(Zi), \+ ploua(Zi). % nu se poate demonstra ca ploua(Zi)
	% :- = "dacă"  (seamana cu <= )
	% , = si
	% ; = sau


ploua(maine).
ploua(intai_mai).

zi(maine).
zi(poimaine).
zi(intai_mai).
zi(doi_mai).
zi(treizeci_aprilie).





% Alice came across a lion and a unicorn in a forest of forgetfulness.
% Those two are strange beings.
% The lion lies every Monday, Tuesday and Wednesday
%	and the other days he speaks the truth.
% The unicorn lies on Thursdays, Fridays and Saturdays,
%	however the other days of the week he speaks the truth.
% Lion: Yesterday I was lying.
% Unicorn: So was I.
% Which day did they say that?


days([mon, tue, wed, thu, fri, sat, sun]).

% is_next/3
% is_next(+Day, +NextDay, +DaysList)
is_next(D1, D2, [D1, D2 | _]).
is_next(D1, D2, [_|RestDays]) :- is_next(D1, D2, RestDays).



yesterday(Today, Yesterday) :-
	days(Days),
	member(Today, Days),
	member(Yesterday, Days),
	is_next(Yesterday, Today, Days).
yesterday(mon, sun).




% myMember(?E, +L)
myMember(E, [E|_]).
%myMember(E, L) :- [H|T]=L, myMember(E, T).
myMember(E, [_|T]) :- myMember(E, T).


% first_two(+L, -E1, -E2)
%first_two(L, E1, E2) :- [E1, E2 | _]=L.
first_two([E1, E2 | _], E1, E2).


sumList([], 0).
sumList([H|T], Sum) :- sumList(T, SumTail), Sum is H + SumTail.