1. A system for improved simulation of a biological system comprising a plurality of chemical reactions, the system comprising: a storage; and a processor for: constructing a graphical model of a biological system, the graphical model of the biological system comprising at least one chemical reaction, the at least one chemical reaction represented using a wild card character that allows multiple instances of an expression to be identified using a single reaction, and a merge block having an input and an output, the output of the merge block at a time step during an execution of the graphical model being equal to a most recent input provided to the merge block, accepting as input the constructed graphical model of the biological system, and executing the graphical model to generate as output dynamic behavior of the biological system by using the merge block and the at least one chemical reaction to produce a set of reactions described by the multiple instances of the expression.