Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Hind : ∀ (H := CoreTactics.block) (l3 l4 l : list A),
           {| pr1 := l1; pr2 := l2 |} = {| pr1 := l3; pr2 := l4 |}
           → split_prefix l1 l2 = split_prefix l3 l4
             → let H0 := CoreTactics.block in
               is_prefix l l3
               → is_prefix l l4
                 → is_prefix l (fst (fst (split_prefix l3 l4)))
  Heq0 : (a1 == a2) = true
  l : list A
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
fail 1

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

    A : Type  H : ReflectEq A  a : A  l : list A  l0 : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l0 (a :: l)
    → is_prefix l0 [] → is_prefix l0 (fst (fst (split_prefix (a :: l) []))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l : list A
  Heqcall : ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Hind : ∀ (H := CoreTactics.block) (l3 l4 l : list A),
           {| pr1 := l1; pr2 := l2 |} = {| pr1 := l3; pr2 := l4 |}
           → split_prefix l1 l2 = split_prefix l3 l4
             → let H0 := CoreTactics.block in
               is_prefix l l3
               → is_prefix l l4
                 → is_prefix l (fst (fst (split_prefix l3 l4)))
  Heq0 : (a1 == a2) = true
  l : list A
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
cbn beta iota
            delta [Prop.Logic.transport Logic.eq_elim Logic.eq_elim_r eq_rect
                  DepElim.pack_sigma_eq DepElim.pack_sigma_eq_nondep] 
            in *
Level 3: Backtrace:

Tactic failure (level 1).
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.

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

    A : Type  H : ReflectEq A  a : A  l : list A  l0 : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l0 (a :: l)
    → is_prefix l0 [] → is_prefix l0 (fst (fst (split_prefix (a :: l) []))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l : list A
  Heqcall : ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Hind : ∀ (H := CoreTactics.block) (l3 l4 l : list A),
           {| pr1 := l1; pr2 := l2 |} = {| pr1 := l3; pr2 := l4 |}
           → split_prefix l1 l2 = split_prefix l3 l4
             → let H0 := CoreTactics.block in
               is_prefix l l3
               → is_prefix l l4
                 → is_prefix l (fst (fst (split_prefix l3 l4)))
  Heq0 : (a1 == a2) = true
  l : list A
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
DepElim.simplify_IH_hyps

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

    A : Type  H : ReflectEq A  a : A  l : list A  l0 : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l0 (a :: l)
    → is_prefix l0 [] → is_prefix l0 (fst (fst (split_prefix (a :: l) []))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l : list A
  Heqcall : ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))

  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Hind : ∀ (H := CoreTactics.block) (l3 l4 l : list A),
           {| pr1 := l1; pr2 := l2 |} = {| pr1 := l3; pr2 := l4 |}
           → split_prefix l1 l2 = split_prefix l3 l4
             → let H0 := CoreTactics.block in
               is_prefix l l3
               → is_prefix l l4
                 → is_prefix l (fst (fst (split_prefix l3 l4)))
  Heq0 : (a1 == a2) = true
  l : list A
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
repeat
            match goal with
            | hyp:context [ CoreTactics.block ]
              |- _ =>
                  cbn beta in hyp; eqns_specialize_eqs_block hyp;
                   cbn[eq_rect_r eq_rect] in hyp
            end