Goal:
  
  A : Type
  H : ReflectEq A
  l1 : list A
  l2 : list A
  l : list A
  packcall := {| pr1 := l1; pr2 := l2 |} : sigma (λ _ : list A, list A)
  packcall_fn := {|
                   pr1 := packcall;
                   pr2 := split_prefix (pr1 packcall) (pr2 packcall)
                 |}
              : sigma
                  (λ _ : sigma (λ _ : list A, list A),
                     (list A * list A * list A)%type)
  eqargs : packcall = {| pr1 := l1; pr2 := l2 |}
  Heqcall : split_prefix (pr1 packcall) (pr2 packcall) =
            split_prefix (pr1 {| pr1 := l1; pr2 := l2 |})
              (pr2 {| pr1 := l1; pr2 := l2 |})
  ============================
   (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:
tac H