The domain specific modeling and simulation language ML-Rules makes it possible to describe cell biological systems at different levels of organization. A model is formed by attributed and dynamically nested species, with reactions that are constrained by functions on attributes. In this paper, we extend ML-Rules to also support constraints using functions on multi-sets of species, i.e., solutions. Further, we present the formal syntax and semantics of ML-Rules, we define its stochastic simulator and we illustrate its expressiveness based on a model of the cell cycle and proliferation.