reserve MS for satisfying_equiv MusicStruct;
reserve a,b,c,d,e,f for Element of MS;
reserve MS for satisfying_octave_descendent_constructible
  classical_octave satisfying_octave_constructible classical_fifth
  satisfying_fifth_constructible satisfying_harmonic_closed satisfying_Nat
  satisfying_commutativity satisfying_interval satisfying_equiv satisfying_Real
  non empty MusicStruct,
  fondamentale,frequency for Element of MS;
reserve                              MS for MusicSpace,
        fondamentale, frequency, f1, f2 for Element of MS;

theorem Th79:
  REAL_Music is satisfying_fourth_constructible
  proof
  set MS = REAL_Music;
  let frequency be Element of MS;
  consider fr,qr be positive Real such that
A1: fr = frequency and
  qr = (4 qua Real) / 3 * fr and
A2: [fr,qr] in fourth(MS) by Th78;
  fr is Element of MS & qr is Element of MS by Th1;
  hence thesis by A1,A2;
end;
