Prolog: append is failing inside loop -

i have following prolog code:

my_function(a):-    my_function(a,[],1).  my_function(a,list,num):-    num_new num+1,    generatel(o,s,l),    write(l),nl,    %append(l,list,list_new),    (  num_new < 20 -> my_function(a,list_new,num_new)  ). 

which working. loops through 19 times, calls generatel generates new term l print screen.

i want on each iteration append l list list create new list list_new passed through in recursive loop list such in next iteration list l again appended to.

i have attempted writing append statement commented out above. problem statement failing - believe because list_new not seen variable, rather pre-existing list, append testing whether list_new indeed l , list appended together, opposed creating new list list_new.

any ideas on how solve this? thanks!

if l term, can replace:

 %append(l,list,list_new), 

with:

 list_new=[l|list],