predicates
%    pair(symbol,symbol)
     iskiller(symbol,symbol)
    male(symbol)
    female(symbol)
    isvictim(symbol)
    not_at_bar(symbol,symbol)
    not_at_beach(symbol,symbol)
    not_alone(symbol)
    twin(symbol,symbol)
    younger(symbol,symbol)
    child(symbol)
    
clauses    
    male(husband).
    male(brother).
    male(son).
    
    female(alice).
    female(daughter).
    
    twin(brother,alice).
    twin(son,daughter).
    
    child(son).
    child(daughter).
    
    not_alone(
    not_alone(alice).
    not_alone(brother).
    not_alone(X):-
        
        
    child(X),child(Y)
        
    
    not_at_beach(husband,alice).
    not_at_beach(son,daughter).
    
    not_at_bar(son,daughter).
    not_at_bar(husband,alice).
    not_at_bar(X,Y):-
        male(X),male(Y).
    not_at_bar(X,Y):-
        female(X),female(Y).
    
    
    isvictim(X):-
        twin(X,Y),not(iskiller(Y,X)).
    isvictim(X):-
        twin(Y,X),not(iskiller(Y,X)).
                
        
    younger(son,alice).
    younger(son,husband).
    younger(daughter,alice).
    younger(daughter,husband).

    iskiller(X,Y):-
        not(alone(X)),
        younger(X,Y),
        not(not_at_beach(X,Y)),
        not(not_at_bar(X,Y)).