theorem
  S is (R2) (R4) implies (a")" * 1.S <<>> a
  proof
    assume
A1: S is (R2) (R4);
    take (a")" * (a" * a);
    a" * a ==> 1.S by A1;
    hence (a")" * (a" * a) =*=> (a")" * 1.S by Th2,ThI3;
    thus (a")" * (a" * a) =*=> a by A1,Th2;
  end;
