Lean4
instance instLinearOrder : LinearOrder PUnit where
le := fun _ _ ↦ True
lt := fun _ _ ↦ False
max := fun _ _ ↦ unit
min := fun _ _ ↦ unit
toDecidableEq := inferInstance
toDecidableLE := fun _ _ ↦ Decidable.isTrue trivial
toDecidableLT := fun _ _ ↦ Decidable.isFalse id
le_refl := by intros; trivial
le_trans := by intros; trivial
le_total := by intros; exact Or.inl trivial
le_antisymm := by intros; rfl
lt_iff_le_not_ge := by simp only [not_true, and_false, forall_const]