Goal:
  
  A : Type
  P : list A → A → Type
  l : list A
  a : All_fold P l
  b : All_fold P l
  H : a = b
  ============================
   (NoConfusionHom_All_fold A P l a b)


DEBUG:Going to execute:
lazymatch goal with
           | H:_ |- _ => tac H
           end