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<div class="nb-cell markdown" name="md1">
Answering stackoverflow question https://stackoverflow.com/q/47911117/874024
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<div class="nb-cell program" name="p1">
:- use_module(library(clpfd)).

skyscrape_row(Left, Right, Heights) :-
    constraint_view(0, Heights, LHeights),
    sum(LHeights, #=, Left),
    reverse(Heights, Heights_),
    constraint_view(0, Heights_, RHeights),
    sum(RHeights, #=, Right).

constraint_view(_, [], []).
constraint_view(Top, [V|Vs], [R|Rs]) :-
    R #&lt;==&gt; V #&gt; 0 #/\ V #&gt; Top,
    Max #= max(Top, V),
    constraint_view(Max, Vs, Rs).

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<div class="nb-cell query" name="q1">
L=[A,B,C,D], L ins 1..4, all_different(L), skyscrape_row(2,3,L), label(L).
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