(motive : Lean.Meta.Grind.Arith.CommRing.RingExpr → Sort u_1) →
  (rb : Lean.Meta.Grind.Arith.CommRing.RingExpr) →
    (Unit → motive (Lean.Grind.CommRing.Expr.num 0)) →
      ((x : Lean.Meta.Grind.Arith.CommRing.RingExpr) → motive x) → motive rb