theorem
  S is (R6) (R7) implies a * 1.S <<>> a
  proof
    assume
A1: S is (R6) (R7);
    take a""*1.S;
    thus a""*1.S =*=> a*1.S by A1,Th2;
    thus a"" * 1.S =*=> a by A1,Th2;
  end;
