Goal:
  
  A : Type
  P : list A → A → Type
  b : All_fold P []
  H : NoConfusionHom_All_fold A P [] (All_fold_nil P) b
  ============================
   (let H := CoreTactics.block in All_fold_nil P = b)


DEBUG:Going to execute:
match id' with
           | id => idtac
           | _ => revert id'; CoreTactics.revert_until id
           end