Neighbors

:- use_module(library(clpfd)).

valid_neighbors(_, [_]).

valid_neighbors(Size, [A, N, B|T]):-
    N = x,
    A #> 0,
    A #=< Size,
    (A #= B - 1;
     A #= B + 1),
    valid_neighbors(Size, [B|T]).

valid_neighbors(Size, [A, N, B|T]):-
    N = o,
    A #> 0,
    A #=< Size,
    A #\= B - 1,
    A #\= B + 1,
    valid_neighbors(Size, [B|T]).

% Sample Query
?- valid_neighbors(4, [A, x, B, o, C, x, D]).

% Output
A in 1..3,
A+1#=B,
B in 2..4,
B#\=C+1,
C#\=B+1,
C in 1..4,
C+1#=D,
D in 2..5 % Size is passed as 4, how can D be 5 when we have the constraint A #=< Size?