∀ {R : Type u_1} {K : Type u_2} [inst : CommRing R] [inst_1 : Field K] [inst_2 : Algebra R K] [IsFractionRing R K]
  [inst_4 : IsDomain R] [Subsingleton (ClassGroup R)] (I : Ideal R), IsUnit ↑I → Submodule.IsPrincipal I