theorem
  S is (R1) (R4) implies (1.S)" * a <<>> a
  proof
    assume
A1: S is (R1) (R4);
    take (1.S)"*(1.S*a);
    1.S*a ==> a by A1;
    hence (1.S)"*(1.S*a) =*=> (1.S)" * a by Th2,ThI3;
    thus thesis by A1,Th2;
  end;
