Difference series of sequence:
abjad> from abjad.tools import mathtools
abjad> mathtools.difference_series([1, 1, 2, 3, 5, 5, 6])
[0, 1, 1, 2, 0, 1]
Return list.
Changed in version 2.0: renamed sequencetools.difference_series() to mathtools.difference_series().