/* the empty list is denote by [] */
/* [x,y] denotes a list of two elements */
/* [H|T] is a list of possibly many elements where H is the head and T is the rest of the list */

append2([],X,X).  /*base case*/
append2([H|T], X,[H|L]) :- append2(T,X,L). /**/

twoelement([A,B],A,B).  /* say yes to two element lists where H is the first element and T is the second */


rev2([],[]). /*base case*/
rev2([H|T],X) :- rev2(T,Y),append2(Y,[H],X).