Goal:
  
  A : Type
  P : list A → A → Type
  b : All_fold P []
  H : NoConfusionHom_All_fold A P [] (All_fold_nil P) b
  ============================
   (All_fold_nil P = b)


DEBUG:Going to execute:
match goal with
           | H':_ |- _ => match H' with
                          | H => idtac
                          end
           end