Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  b : B
  l' : list B
  H2 : ∀ (x : A) (y : B), In y (b :: l') → P x → Q x y
  Hl1 : nat
  Hl : S #|l'| = Hl1
  ============================
   (0 = Hl1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 Q [] (b :: l'))


DEBUG:Going to execute:
cbv beta iota delta [TODO projection TODO projection] in id, id'
DEBUG LTAC Evaluated term: Hl1
DEBUG LTAC Evaluated term: nat
Level 3: type of id
 has value 
nat