A.24 library(lists): List Manipulation
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Availability::- use_module(library(lists)).(can be autoloaded)
Source[det]nth0(?N, ?List, ?Elem, ?Rest)
Select/insert element at index. True when Elem is the N’th (0-based) element of List and Rest is the remainder (as in by select/3) of List. For example: 
?- nth0(I, [a,b,c], E, R).
I = 0, E = a, R = [b, c] ;
I = 1, E = b, R = [a, c] ;
I = 2, E = c, R = [a, b] ;
false.
?- nth0(1, L, a1, [a,b]).
L = [a, a1, b].