sorting - Prolog Bubblesort -

i'm trying best understand code. how perceive what's happening:

so can see if x > y swap elements, if not recurse down sublist until find x x > y, if not, list sorted. problems i'm having don't understand base case, bubblesort(sorted, sorted). thought need base case empty list? appreciate if describe sort of step step description of program.

bubblesort(list,sorted) :-    swap(list,list1),    !,    bubblesort(list1,sorted). bubblesort(sorted,sorted).  swap([x,y|rest],[y,x|rest]) :-   % swaps x y if gt(x,y) true.    gt(x,y). swap([z|rest],[z|rest1]) :-      % calls swap on sublists excluding heads.    swap(rest,rest1).  gt(x,y) :-                       % true if x greater y.    x > y.