+ /**
+ * @brief Computes the backtrack set for this state
+ * according to its definition in Simgrid.
+ *
+ * The backtrack set as it appears in DPOR, SDPOR, and ODPOR
+ * in SimGrid consists of those actors marked as `todo`
+ * (i.e. those that have yet to be explored) as well as those
+ * marked `done` (i.e. those that have already been explored)
+ * since the pseudcode in none of the above algorithms explicitly
+ * removes elements from the backtrack set. DPOR makes use
+ * explicitly of the `done` set, but we again note that the
+ * backtrack set still contains processes added to the done set.
+ */
+ std::unordered_set<aid_t> get_backtrack_set() const;