% Part 3: More Robust Rules & Queries
% Question 3.1: connected

%definition of connected/2
connected(U1,U2):- connected(U1,U2,[]).
%definition of connected/3
connected(U1,U2,_):- user(U1),user(U2),connection(U1,U2,_).
connected(U1,U3,Us):-
    user(U1), user(U2), connection(U1,U2,_),
    user(U3), not(member(U2,Us)), not(member(U3,Us)), U1\=U3,
    connected(U2,U3,[U2,U3|Us]).