let enlarge_to_left ~extend_left same_values bi new_vv acc =
if extend_left then
(* look for an interval ending just before i *)
let p_bi = Int.pred bi in
match concerned_intervals Int_Interv.fuzzy_order (p_bi,p_bi) acc
with [] -> acc,bi
| [(ba,ea) as a,vva] -> assert (Int.equal ea p_bi);
if same_values vva new_vv then
(remove a acc),ba
else acc,bi
| _ -> assert false
else acc, bi