Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  packcall : let H := split_prefix in sigma (λ _ : list A, list A)
  ============================
   (packcall = {| pr1 := l1; pr2 := l2 |}
    → split_prefix (pr1 packcall) (pr2 packcall) =
      split_prefix (pr1 {| pr1 := l1; pr2 := l2 |})
        (pr2 {| pr1 := l1; pr2 := l2 |})
      → let H := CoreTactics.block in
        let
        '(prefix, l1', l2') := split_prefix l1 l2 in
         l1 = prefix ++ l1' ∧ l2 = prefix ++ l2')


DEBUG:Going to execute:
with_last_secvar_aux tac
Level 2: evaluation returns
with_last_secvar_aux tac + orelse
where
tac := fun eos => move packcall before eos
       where
       tac := fun _ => idtac
              where
              c := split_prefix l1 l2
              of type
              constr
       of type
       tacvalue
       packcall_fn := packcall_fn
       of type
       intropattern
       packcall := packcall
       of type
       intropattern
       elimn := 5
       of type
       constr
       elimfn := split_prefix
       of type
       constr
       elimc := fun_elim
       of type
       constr
       c := split_prefix l1 l2
       of type
       constr
       Heq := Heqcall
       of type
       intropattern
       H := eqargs
       of type
       intropattern
of type
tacvalue
orelse := move packcall at top
          where
          tac := fun _ => idtac
                 where
                 c := split_prefix l1 l2
                 of type
                 constr
          of type
          tacvalue
          packcall_fn := packcall_fn
          of type
          intropattern
          packcall := packcall
          of type
          intropattern
          elimn := 5
          of type
          constr
          elimfn := split_prefix
          of type
          constr
          elimc := fun_elim
          of type
          constr
          c := split_prefix l1 l2
          of type
          constr
          Heq := Heqcall
          of type
          intropattern
          H := eqargs
          of type
          intropattern
of type
tacvalue of type tacvalue