New in version 2.0.
Yield all unordered pairs of sequence:
abjad> from abjad.tools import sequencetools
abjad> list(sequencetools.yield_all_unordered_pairs_of_sequence([1, 2, 3, 4]))
[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
Yield all unordered pairs of length-1 sequence:
abjad> list(sequencetools.yield_all_unordered_pairs_of_sequence([1]))
[]
Yield all unordered pairs of empty sequence:
abjad> list(sequencetools.yield_all_unordered_pairs_of_sequence([]))
[]
Yield all unordered pairs of sequence with duplicate elements:
abjad> list(sequencetools.yield_all_unordered_pairs_of_sequence([1, 1, 1]))
[(1, 1), (1, 1), (1, 1)]
Pairs are tuples instead of sets to accommodate duplicate sequence elements.
Return generator.