:- pl( swi(_), true ).
:- pl( yap(_), ensure_loaded(library(terms)) ).
:- pl( sicstus(_), ensure_loaded(library(terms)) ).

list_frequency( [H|T], [H-HTimes|CountedT] ) :-
        list_frequency_1( T, H, 1, HTimes, ReducedT ),
        list_frequency( ReducedT, CountedT ).
list_frequency( [], [] ).

list_frequency_1( [H|T], El, Acc, Count, RedT ) :-
        variant( El, H ),
        !,
        Acc1 is Acc + 1,
        list_frequency_1( T, El, Acc1, Count, RedT ).
list_frequency_1( [H|T], El, Acc, Count, [H|RedT] ) :-
        list_frequency_1( T, El, Acc, Count, RedT ).
list_frequency_1( [], _El, Acc, Acc, [] ).