- "we manually weighted failure actions to reduce the likelihood of them being chosen"
- "we implemented depth-first search (DFS) in TLC"
- "TLC had already been enhanced to support trace validation"
- In the TLA+ debugger, "we implemented a new unsatisfied breakpoint that activates for each state in T that is found to be unreachable"
- "T can be visualized as a graph that not only includes all unreachable states but also references the subformula responsible for the state being unreachable"
The paper summarizes this effort as "enhancing the TLC model checker to support trace validation, which involved implementing support for action composition, DFS, improved debugging support, and visualizing the state graph." So I think the enhancements are in both TLC and the VS Code TLA+ debugger.