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

theorem Th50:
  RAT_Music is satisfying_octave_descendent_constructible
  proof
    let frequency be Element of RAT_Music;
    consider fr,qr be positive Rational such that
A1: fr = frequency and
    qr = (1 qua Rational) / 2 * fr and
A2: [qr,fr] in octave(RAT_Music) by Th49;
    fr is Element of RAT_Music & qr is Element of RAT_Music by Th2;
    hence thesis by A1,A2;
  end;
