open tactic

axiom Sorry : ∀ A : Type, A

inductive Enum : Type | ea | eb | ec | ed
attribute [instance]
noncomputable definition Enum_dec_eq : decidable_eq Enum :=
by do a ← intro `a, cases a,
      b ← intro `b, cases b,
      right >> reflexivity,
      try (do left, h ← intro `H, cases h),
      repeat $ intros >> mk_const `Sorry >>= apply