Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  b : All2_dep R' All2_nil
  ============================
   (NoConfusionHom_All2_dep A B R R' [] [] All2_nil All2_dep_nil b
    → let H := CoreTactics.block in All2_dep_nil = b)


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