Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  l : list A
  ll1 : is_prefix (rev l) (rev l1)
  ll2 : is_prefix (rev l) (rev l2)
  ============================
   (is_prefix (rev l) (fst (fst (split_prefix (rev l1) (rev l2))))
    → is_prefix (rev l)
        (rev
           (snd
              (let
               '(suffix, l1, l2) := split_prefix (rev l1) (rev l2) in
                (rev l1, rev l2, rev suffix)))))


DEBUG:Going to execute:
case : (split_prefix _ _)  => [[? ?] ?] /=