Conjunctive grammars with restricted disjunction

It is shown that every conjunctive language is generated by a conjunctive grammar from a special subclass, in which every nonterminal A has at most one rule of the general form A→α1&…&αn, while the rest of the rules for A must be of the type A→w, where w is a terminal string. For context-free grammars, a similar property does not hold (S.A. Greibach et al. (1992) [3]). If it is furthermore required that each rule A→w has a nonempty w, then a substantial subfamily of conjunctive languages can be generated, yet it remains unknown whether such grammars are as powerful as conjunctive grammars of the general form.