  This paper solves an open problem concerning the generative power of
nonerasing context-free rewriting systems using a simple mechanism for checking
for context dependencies, in the literature known as semi-conditional grammars
of degree (1,1). In these grammars, two nonterminal symbols are attached to
each context-free production, and such a production is applicable if one of the
two attached symbols occurs in the current sentential form, while the other
does not. Specifically, this paper demonstrates that the family of languages
generated by semi-conditional grammars of degree (1,1) coincides with the
family of random context languages. In addition, it shows that the normal form
proved by Mayer for random context grammars with erasing productions holds for
random context grammars without erasing productions, too. It also discusses two
possible definitions of the relation of the direct derivation step used in the
literature.
