refresh(_Concept) :-
	known(_Concept, _Value),
	ask_usr(_Concept, _Value).
refresh(_).					%% always succeed
refresh(_Concept, _Keys) :-
	known(_Concept, _Keys, _Value),
	ask_usr(_Concept, _Value).
refresh(_, _).					%% always succeed

%%  append(First_part, Second_part, List) iff List is the
%%  concatenation of the first two arguments.

append([], List, List).
append([Elem | First_part], Second_part, [Elem | List]) :-
  append(First_part, Second_part, List).

