{motive : Lean.IR.Decl → Sort u} →
  (t : Lean.IR.Decl) →
    t.ctorIdx = 1 →
      ((f : Lean.IR.FunId) →
          (xs : Array Lean.IR.Param) →
            (type : Lean.IR.IRType) → (ext : Lean.ExternAttrData) → motive (Lean.IR.Decl.extern f xs type ext)) →
        motive t