LDT: a language definition technique

We establish a semantics for building grammars from a modularised specification in which modules are able to delete individual productions from imported nonterminals. Our goal is to allow precise answers to the question: 'what character level language does this grammar generate' in the face of difficult issues such as the mutual embedding of languages that have different whitespace and commenting conventions. Our technique is to automatically generate a character level grammar from grammars written at token level in the conventional way; the grammars is constructed from modules each of which may have its own whitespace convention. Modules have import and export lists, and productions may be deleted nonterminals upon import. We conclude with a discussion of a concrete syntax which we use as the input language to our parser generator.