Here we define the weight for edge \$$e(t,x,x')\$$:

* Weight(e(t,x,x')) = RateConstant(t) * \$$\prod_{\mathit{species} \in S}\$$ FallingPower(Population(x,species), InpDegree(t, species))

Where:

* FallingPower(n, k) = n * (n-1) * ... * (n - k + 1)

This gives the "rate of probability flow" through the edge.