sort/2sort(+List1, -List2)
Sorts the elements of the list List1 into the ascending standard order, and removes any multiple occurrences of an element. The resulting sorted list is unified with the list List2.
instatiation_errortype_error | ?- sort([a,X,1,a(x),a,a(X)], L).
L = [X,1,a,a(X),a(x)]
(The time taken to do this is at worst order (N log N) where N is the length of the list.)