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Compute distance spectrum of convolutional code

computes the free distance and the requested number of components of the weight and distance
spectra of a linear convolutional code. Because convolutional codes do not have block
boundaries, the weight spectrum and distance spectrum are semi-infinite and well
approximated by the first few components.`spect`

= distspec(`trellis`

,`numcomp`

)

The function uses a tree search algorithm implemented with a stack, as described in [2].

[1] Bocharova, I.E., and B.D. Kudryashov. “Rational Rate Punctured
Convolutional Codes for Soft-Decision Viterbi Decoding.” *IEEE
Transactions on Information Theory* 43, no. 4 (July 1997): 1305–13.
https://doi.org/10.1109/18.605600.

[2] Cedervall, M.L., and R. Johannesson. “A Fast Algorithm for
Computing Distance Spectrum of Convolutional Codes.” *IEEE
Transactions on Information Theory* 35, no. 6 (November 1989): 1146–59.
https://doi.org/10.1109/18.45271.

[3] Chang J., D. Hwang, and M. Lin. “Some Extended Results on the
Search for Good Convolutional Codes.” *IEEE Transactions on
Information Theory* 43, no. 5 (September 1997): 1682–97.
https://doi.org/10.1109/18.623175.

[4] Frenger, P.K., P. Orten, and T. Ottosson. “Comments and
Additions to Recent Papers on New Convolutional Codes.” *IEEE Transactions on
Information Theory* 47, no. 3 (March 2001): 1199–1201.
https://doi.org/10.1109/18.915683.