nth1( A, B, C ) :-
	( number( A ) -> 
     	nth( A, B, C ) 
		;
		var( A ),
		nth1_gen_yap( B, 1, A, C )
	).

nth1_gen_yap( [H|T], Acc, Acc, H ).
nth1_gen_yap( [_H|T], Acc, N, Nth ) :-
	NxAcc is Acc + 1,
	nth1_gen_yap( T, NxAcc, N, Nth ).

nth1( A, B, C, D ) :-
	( number( A ) -> 
     	nth( A, B, C, D ) 
		;
		var( A ),
		nth1_gen_yap( B, 1, A, C, D )
	).

nth1_gen_yap( [H|T], Acc, Acc, H, T ).
nth1_gen_yap( [H|T], Acc, N, Nth, [H|Rest] ) :-
	NxAcc is Acc + 1,
	nth1_gen_yap( T, NxAcc, N, Nth, Rest ).