Prolog - Exercise on facts and lists

Question

The problem is that I need to program a predicate capable of consulting the products in a database so that it returns a list filled with the names of the products its prices are reduced(that is indicated by the "state".

domains
    state =  reduced ; normal
    element = string
    list = element*
database
    producte (string, integer, state)
predicates
    nondeterm reduced(list)
clauses 
   % ---> producte( description , price , state )
    producte("Enciam",2,reduced). 
producte("Peix",5,normal).
producte("Llet",1,reduced). 
producte("Formatge",5,normal).
    %unique case
    reduced([D]):-
         producte(D,_,reduced).
    %general case
    reduced([D|L]) :- 
         producte(D,_, reduced),retract(producte(D,_,reduced)),reduced(L).
Goal
    reduced(List).

I appreciate it. Now, it gives me three different solutions. How could I force the predicate to give me one solution? In fact, The last one?

Answer 1

Since I don't use visual-prolog, I'll just propose something I found in the doc.

reduced(List) :-
    List = [ Price || producte(_, Price, reduced) ].

Answer 2

What about when the first product in the list is NOT reduced -- you have no rule for that case.