Hi
In fact, in the code, the treatment of lateral inflow and lateral weir is done in the same step but only under the condition of the number of inflow is positive. So if you add a lateral inflow in you model, you will activate this part of code and then the lateral weir will work.
the model should look like this:
In the building of the network, the lateral weir looks like an arrow going from the network to the outside, the lateral inflow is the contrary (an arrow coming from the outside to the network). As you could choose the discharge law, if you create a law with Q=0, this will doesn't change your model...
In steady flow, the part of discharge which pass the lateral weir in simply "lost" for the simulation.
The result will look like this:
Hope this helps