Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  x : A
  y : B
  z : C
  l : list A
  l' : list B
  l'' : list C
  r : R x y z
  a : All3 R l l' l''
  b : All3 R (x :: l) (y :: l') (z :: l'')
  ============================
   (NoConfusionHom_All3 A B C R (x :: l) (y :: l') 
      (z :: l'') (All3_cons r a) b
    → let H := CoreTactics.block in All3_cons r a = b)


DEBUG:Going to execute:
CoreTactics.block_goal
Level 11: evaluation returns
idtac of type tacvalue

Level 9: evaluation returns
idtac of type tacvalue

Level 8: evaluation returns
idtac of type tacvalue

Level 6: evaluation returns
idtac of type tacvalue

Level 4: evaluation returns
idtac of type tacvalue

Level 3: evaluation returns
idtac of type tacvalue