Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  Q : A → B → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : P x y
  X : All2 P l l'
  IHX : All2 Q l l' → All2 (Trel_conj P Q) l l'
  H : All2 Q (x :: l) (y :: l')
  ============================
   (let H := CoreTactics.block in All2 (Trel_conj P Q) (x :: l) (y :: l'))


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end