I'm reading
this paper and in section 2.1 it defines a reaction as "an element [of a list] $\rho = (\mathbf{r},\mathbf{p}) \in \mathbb{N}^{|Lambda}\times\mathbb{N}^{\Lambda}$ [where $\Lambda$ is a finite set of species (basically chemicals)], where $\mathbf{r}$ and $\mathbf{p}$ specify the stoichiometry of the reactants and products, respectively." It later gives an example