New in version 2.0.
List harmonic diatonic intervals in expr:
abjad> staff = Staff("c'8 d'8 e'8 f'8")
abjad> for interval in sorted(pitchtools.list_harmonic_diatonic_intervals_in_expr(staff)):
... interval
...
HarmonicDiatonicInterval('m2')
HarmonicDiatonicInterval('M2')
HarmonicDiatonicInterval('M2')
HarmonicDiatonicInterval('m3')
HarmonicDiatonicInterval('M3')
HarmonicDiatonicInterval('P4')
Return unordered set.