Chapter 10 Regulated ET0L Grammars

ET0L grammars can be seen as generalized parallel versions of context-free grammars. More precisely, there exist three main conceptual differences between them and context-free grammars. First, instead of a single set of rules, they have finitely many sets of rules. Second, the left-hand side of a rule may be formed by any grammatical symbol, including a terminal. Third, all symbols of a string are simultaneously rewritten during a single derivation step. The present chapter studies ET0L grammars regulated in a context-conditional way. Specifically, by analogy with sequential context-conditional grammars, this chapter discusses context-conditional ET0L grammars that capture this dependency so each of their rules may be associated with finitely many strings representing permitting conditions and, in addition, finitely many strings representing forbidding conditions. A rule like this can rewrite a symbol if all its permitting conditions occur in the rewritten current sentential form and, simultaneously, all its forbidding conditions do not. Otherwise, these grammars work just like ordinary ET0L grammars. The chapter consists of four sections. Section 10.1 defines the basic version of context-conditional ET0L grammars. The other sections investigate three variants of the basic version—forbidding ET0L grammars (Sect. 10.2), simple semi-conditional ET0L grammars (Sect. 10.3), and left random context ET0L grammars (Sect. 10.4). All these sections concentrate their attention on establishing the generative power of the ET0L grammars under investigation.