Issue No. 08 - August (1970 vol. 19)
S. Bergman , Temple University
This paper concerns itself with rooted trees which have labeled nodes. The labels are taken from a stratified alphabet (each label is associated with a nonnegative number, the number of branches descending from it). The alphabet is divided into terminal and nonterminal labels. Tree generating grammars called regular systems are introduced. A set of trees serve as "axioms" and production rules of the form F??, where F and F are trees, allow successive replacement of subtrees F by ?. The "language" generated by such a system is the set of trees generable from the axioms containing terminal labels only. Such languages, when trees are written linearly (prefix or postfix form) are context free.
S. Bergman, "R70-27 Tree Generating Regular Systems", IEEE Transactions on Computers, vol. 19, no. , pp. 764, August 1970, doi:10.1109/T-C.1970.223032