Truncate subruns of like elements in l to length 1:
abjad> l = [1, 1, 2, 3, 3, 3, 9, 4, 4, 4]
abjad> listtools.truncate_subruns(l)
[1, 2, 3, 9, 4]
Return empty list when l is empty:
abjad> listtools.truncate_subruns([ ])
[]
Raise TypeError when l is not a list:
abjad> listtools.truncate_subruns(1)
TypeError