Make d15p2 a little more readable and remove unneeded imports.

15/part1.pl

@@ -1,5 +1,4 @@
 :- use_module(library(pio)).
-:- use_module(library(clpfd)).
 :- initialization(main, main).
 
 main([FileName|_]) :-

15/part2.pl

@@ -1,20 +1,19 @@
 :- use_module(library(pio)).
 :- use_module(library(dcg/basics)).
-:- use_module(library(clpfd)).
 :- initialization(main, main).
 
 main([FileName|_]) :-
     input(FileName, Actions),
     perform_all(Actions, Boxes),
-    findall(
-        Power,
-        (   nth1(BoxI, Boxes, _-Box),
-            nth1(LensI, Box, _-Focal),
-            Power is BoxI * LensI * Focal),
-        Powers),
+    findall(Power, power_of_some_lens(Boxes, Power), Powers),
     sum_list(Powers, Answer),
     writef('Answer=%t\n', [Answer]).
 
+power_of_some_lens(Boxes, Power) :-
+    nth1(BoxI, Boxes, _-Box),
+    nth1(LensI, Box, _-Focal),
+    Power is BoxI * LensI * Focal.
+
 perform_all(Actions, FinalBoxes) :-
     length(EmptyBoxes, 256),
     foldl([N-[], N, NextN]>>(NextN is N + 1), EmptyBoxes, 0, _),
@@ -29,8 +28,8 @@ remove(_, [], []).
 remove(Label, [Label-_|Box], Box) :- !.
 remove(Label, [Lens|Box], [Lens|NewBox]) :- remove(Label, Box, NewBox).
 
-add(F, Label, OldBox, NewBox) :- select(Label-_, OldBox, Label-F, NewBox), !.
-add(F, Label, OldBox, NewBox) :- append(OldBox, [Label-F], NewBox).
+add(Focal, Label, Box, NewBox) :- select(Label-_, Box, Label-Focal, NewBox), !.
+add(Focal, Label, Box, NewBox) :- append(Box, [Label-Focal], NewBox).
 
 % Input stuff. [0-(add-1)-"rn", 3-remove-"cm", ...]
 input(Name, Actions) :- phrase_from_file(insts(Actions), Name).