theorem
  S is (R4) (R14) implies (a * b)" <<>> b" * a"
  proof
    assume
A1: S is (R4) (R14);
    take b"*(b*(a*b)");
    thus b"*(b*(a*b)") =*=> (a * b)" by A1,Th2;
    (b*(a*b)") ==> a" by A1;
    hence b"*(b*(a*b)") =*=> b" * a" by Th2,ThI3;
  end;
