:- initialization(main).
:- use_module(library(lists)).

main :-
    colors(Places),
    writeq(Places), nl.

colors(Places) :-
    findall(Place-_, neighbours(Place, _), Places),
    places(Places).

places([]).
places([Place-Color|Tail]) :-
    places(Tail),
    neighbours(Place, Neighbours),
    member(Color, [c1, c2, c3, c4]),
    \+ (member(Neighbour-Color, Tail), member(Neighbour, Neighbours)).

neighbours(p1, [p2, p5, p4, p3]).
neighbours(p2, [p1, p4, p3]).
neighbours(p3, [p5, p1, p4, p2]).
neighbours(p4, [p1, p2, p3]).
neighbours(p5, [p1, p3]).