A time-varying distributed H system is a splicing system which has the following feature: at different moments one uses different sets of splicing rules. The number of these sets is called the degree of the system. The passing from a set of rules to another one is specified in a cycle. It is a well known fact that any formal language can be generated by a time-varying distributed H-system of degree at least 7. Here we prove that there are universal time-varying distributed H-systems of degree 2. The question of whether or not there are universal time-varying distributed H-systems of degree 1 remains open.
Margenstern, M., & Rogozhin, Y. (1999). A universal time-varying distributed H-system of degree 2. BioSystems, 52(1–3), 73–80. https://doi.org/10.1016/S0303-2647(99)00034-9