theorem
   for R holds SBFSeri(R) is RingIsomorphism &
   Formal-Series(1,R) is (Formal-Series R)-isomorphic
   proof
     let R;
     SBFSeri(R) is RingHomomorphism & SBFSeri(R) is isomorphism;
     hence thesis by RING_3:def 4;
   end;
