Goal:
    A : Type  B : Type  P : A → B → Type  Q : A → B → Type  H : All2 Q [] []
  ============================
   (let H := CoreTactics.block in All2 (Trel_conj P Q) [] [])


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