Goal:
    A : Type  H : ReflectEq A  l1 : list A  l2 : list A  l : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l l1
    → is_prefix l l2 → is_prefix l (fst (fst (split_prefix l1 l2))))


DEBUG:Going to execute:
uncurry_call elimfn c packcall packcall_fn
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            is_prefix l l1
                            → is_prefix l l2
                              → is_prefix l (fst (fst (split_prefix l1 l2))))