∀ (G : Type u_2) [inst : Group G] {X : Type u_1} [inst_1 : MulAction G X] (a : X)
  (motive : { B // a ∈ B ∧ MulAction.IsBlock G B } → Prop) (x : { B // a ∈ B ∧ MulAction.IsBlock G B }),
  (∀ (val : Set X) (ha : a ∈ val) (hB : MulAction.IsBlock G val), motive ⟨val, ⋯⟩) → motive x