References
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- Herman , G.T. and Walker , A. 1975 . Context-free languages in biological systems . Internat. J. Comput. Math , 4 : 369 – 391 .
- Shyr , H.J. and Thierrin , G. 1974 . “ Preorder relations associated with developmental systems and languages ” . In Automata, Languages, Development , Amsterdam : North Holland .
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