
theorem Th21:
  for MS being satisfying_equiv MusicStruct
  for a,b being Element of MS holds
  a,b equiv a,b
  proof
    let MS be satisfying_equiv MusicStruct;
    let a,b be Element of MS;
    (the Ratio of MS).(a,b) = (the Ratio of MS).(a,b);
    hence thesis by Def08a;
  end;
