New in version 2.0.
Iterate sequence first forward and then backward, with first and last elements repeated:
abjad> from abjad.tools import sequencetools
abjad> list(sequencetools.iterate_sequence_forward_and_backward_nonoverlapping([1, 2, 3, 4, 5]))
[1, 2, 3, 4, 5, 5, 4, 3, 2, 1]
Return generator.