Corollary hist_list_lt : forall h l, hist_list h l -> newer h (length l).

Lemma hist_list'_add : forall h1 h2 (l : list hist_el) (Hdisj : disjoint h1 h2), hist_list' (map_add h1 h2) l -> exists l1 l2, Permutation l (l1 ++ l2) /\ hist_list' h1 l1 /\ hist_list' h2 l2.