  In this paper, we discuss the reduction of error-trellises for tail-biting
convolutional codes. In the case where some column of a parity-check matrix has
a monomial factor (with indeterminate D), we show that the associated
tail-biting error-trellis can be reduced by cyclically shifting the
corresponding error-subsequence by l (the power of D) time units. We see that
the resulting reduced error-trellis is again tail-biting. Moreover, we show
that reduction is also possible using backward-shifted error-subsequences.
