Sum elements in l by sign.
Behavior of sign:
Examples:
abjad> l = [0, 0, -1, -1, 2, 3, -5, 1, 2, 5, -5, -6]
abjad> generator = listtools.sum_by_sign(l)
abjad> generator
<generator object at 0x118bf30>
abjad> list(generator)
[0, -2, 5, -5, 8, -11]
abjad> generator = listtools.sum_by_sign(l, sign = [-1])
abjad> generator
<generator object at 0x118f1e8>
abjad> list(generator)
[0, 0, -2, 2, 3, -5, 1, 2, 5, -11]
abjad> generator = listtools.sum_by_sign(l, sign = [0])
abjad> generator
<generator object at 0x118f288>
abjad> list(generator)
[0, -1, -1, 2, 3, -5, 1, 2, 5, -5, -6]
abjad> generator = listtools.sum_by_sign(l, sign = [1])
abjad> generator
<generator object at 0x118f2d8>
abjad> list(generator)
[0, 0, -1, -1, 5, -5, 8, -5, -6]
abjad> generator = listtools.sum_by_sign(l, sign = [-1, 0])
abjad> generator
<generator object at 0x118f350>
abjad> list(generator)
[0, -2, 2, 3, -5, 1, 2, 5, -11]
abjad> generator = listtools.sum_by_sign(l, sign = [-1, 1])
abjad> generator
<generator object at 0x118f418>
abjad> list(generator)
[0, 0, -2, 5, -5, 8, -11]
abjad> generator = listtools.sum_by_sign(l, sign = [0, 1])
abjad> generator
<generator object at 0x118f508>
abjad> list(generator)
[0, -1, -1, 5, -5, 8, -5, -6]
abjad> generator = listtools.sum_by_sign(l, sign = [-1, 0, 1])
abjad> generator
<generator object at 0x118bf30>
abjad> list(generator)
[0, -2, 5, -5, 8, -11]