Fix point internal hierarchy specification for context free grammars

In the particular case when a context free grammar is used as a model for a computation system, each nonterminal will be naturally associated to a meaning, i.e. a type, and every generation rule represents an operation in the computations system. Thus, the derivation tree (syntax tree) of this grammar yields a hierarchic structure of types as well as a hierarchic structure of system operations. Consequently, on each hierarchic level there exists a set of types and a set of operations, namely a language. Recursive specifications are a useful tool for representing and studying this kind of language hierarchies. As we show in this paper, the recursive specifications are a kind of constructor for these languages.