message: improve the documentation of the flat set
  Summary: As per Arnaud's suggestion in the previous diff.
diff: namespace sparta {
  /*
  - * Represents an unordered set implemented with a sorted vector.
  + * Represents a set implemented with a sorted vector.
  *
  * It is similar to `boost::container::flat_set` but provides set operations
  * such as union, intersection and difference, using the same interface as
  