$($a$)$ Four lines of code in the controller are redacted: write what those lines should contain. Explain the reasoning $($and the pitfalls of simplistic solutions for this problem!$)$

$($b$)$ Draw the reachable subgraph of this controller. Discuss the difference between the full graph and this subgraph, and explain what the subgraph proves.

$($c$)$ For some peculiar reason, the railway company wants to add multiple circuits to this system in the future, so that the bridge is shared by even more trains. How would you restructure the controller for that, and what might be the sources of technical debt in such a system subject to scaling?