illustration of a piece of trajectory crossing the cells $B_\alpha$ with $\alpha=1,\ldots,5$, where cell $B_1$ is identified with the reactant state $A$ and cell $B_5$ with the product state $B$. The circles represent the entry points which would be used to perform sampling restricted in the cells. If only the red points are used as re-entry points, then the restricted sampling selects the pieces of this trajectory when it is assigned to $A$ (these pieces are depicted in red). []{data-label="fig:trajABmiles"}](fig3){width="3in"}

In practice, we do not know explicitly $q_-(\zz)$ (nor even $\varrho(\zz)$ in most nonequilibrium systems) but we can still make use of the observations above to modify the sampling procedure explained before. Suppose that we define the reactant state $A$ as being the union of a group of cells $B_\alpha$ and the product state $B$ as the union of another group. Clearly, what we would then like to do is modify the sampling procedure in such a way that only the re-entry points associated with the trajectory when it is assigned to $A$ are put in the databanks (see the illustration in Fig. \[fig:trajABmiles\]) and keep track of the associated probability fluxes through the boundary of the cells. Indeed, using 