OS systems generalize context-free grammars without non-terminals. It is shown that it is decidable whether or not two arbitrary OS systems generate the same set of (derivation0 sequences. As a corollary we get that it is decidable whether or not two arbitrary context free grammars have the same sets of derivation sequences.
Ehrenfeucht, Andrzej and Rozenberg, Grzegorz, "The Sequence Equivalence Problem Is Decidable for OS Systems ; CU-CS-137-78" (1978). Computer Science Technical Reports. 135.