(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 } → Sort u_3) →
            (x : { B // a ∈ B ∧ MulAction.IsBlock G B }) →
              ((B : Set X) → (ha : a ∈ B) → (hB : MulAction.IsBlock G B) → motive ⟨B, ⋯⟩) → motive x