reserve MS for satisfying_equiv MusicStruct;
reserve a,b,c,d,e,f for Element of MS;

theorem
  for MS being satisfying_harmonique_stable satisfying_harmonic_closed
  satisfying_Nat satisfying_tonic satisfying_interval
  satisfying_equiv MusicStruct for a,b being Element of MS holds a,a equiv b,b
  proof
    let MS be satisfying_harmonique_stable satisfying_harmonic_closed
    satisfying_Nat satisfying_tonic satisfying_interval
    satisfying_equiv MusicStruct;
    let a,b be Element of MS;
    1-harmonique(MS,a) = a & 1-harmonique(MS,b) = b by Th40;
    hence thesis by Def10;
  end;
