Definition term_from_active_term (t: active_term) : term :=
let (f, lv) := t in
fapply f (map (@term_from_value _ _ _) lv).
Definition value_list_order (lv lv' : list value) : Prop :=
suml (map (@value_size _) lv) < suml (map (@value_size _) lv').

Lemma unify_list_aux_extension lv lp s s':
unify_list_aux s lv lp = Some s' ->
exists s'',
s' = s'' ++ s /\
forall x, assoc (variable_beq variable_eq_dec) x s <> None -> assoc (variable_beq variable_eq_dec) x s'' = None.