Goal:
    A : Type  H : ReflectEq A  l1 : list A  l2 : list A  l : list A
  ============================
   (is_prefix l l1
    → is_prefix l l2 → is_prefix l (fst (fst (split_prefix l1 l2))))


DEBUG:Going to execute:
CoreTactics.block_goal; uncurry_call elimfn c packcall packcall_fn;
            remember_let packcall_fn; unfold_packcall packcall