appends two lists ([1,2,3],[4,5]->[1,2,3,4,5]) 
    usage: ?-append([1,2,3],[4,5],X). 
    try also ?-append(A,B,[1,2,3,4,5]). 
      
    append([H|T],L2,[H|R]):-

        append(T,L2,R2). 

    append([],R,R).