chore(set_theory/ordinal/basic): golf ordinal addition definition (#1…

…4744)

src/set_theory/ordinal/basic.lean

@@ -944,8 +944,7 @@ the addition, together with properties of the other operations, are proved in
   every element of `o₁` is smaller than every element of `o₂`. -/
 instance : has_add ordinal.{u} :=
 ⟨λ o₁ o₂, quotient.lift_on₂ o₁ o₂
-  (λ ⟨α, r, wo⟩ ⟨β, s, wo'⟩, ⟦⟨α ⊕ β, sum.lex r s, by exactI sum.lex.is_well_order _ _⟩⟧
-    : Well_order → Well_order → ordinal) $
+  (λ ⟨α, r, wo⟩ ⟨β, s, wo'⟩, by exactI type (sum.lex r s)) $
   λ ⟨α₁, r₁, o₁⟩ ⟨α₂, r₂, o₂⟩ ⟨β₁, s₁, p₁⟩ ⟨β₂, s₂, p₂⟩ ⟨f⟩ ⟨g⟩,
   quot.sound ⟨rel_iso.sum_lex_congr f g⟩⟩