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


DEBUG:Going to execute:
CoreTactics.destruct_sigma b
Level 3: evaluation returns
idtac of type tacvalue