New in version 2.0.
List unordered named chromatic pitch pairs in expr:
abjad> for pair in pitchtools.list_unordered_named_chromatic_pitch_pairs_in_expr(Chord([0, 1, 2, 3], (1, 4))):
... pair
...
(NamedChromaticPitch("c'"), NamedChromaticPitch("cs'"))
(NamedChromaticPitch("c'"), NamedChromaticPitch("d'"))
(NamedChromaticPitch("c'"), NamedChromaticPitch("ef'"))
(NamedChromaticPitch("cs'"), NamedChromaticPitch("d'"))
(NamedChromaticPitch("cs'"), NamedChromaticPitch("ef'"))
(NamedChromaticPitch("d'"), NamedChromaticPitch("ef'"))
Return generator.