code: private int getKey(boolean activePrevious, boolean activeCurrent) {
          if (activePrevious) {
              return activeCurrent ? RETAINED : REMOVED;
          } else {
              // !activePrevious
              return activeCurrent ? ADDED : DEAD;
          }
      }
docs: Get key used to classify the presence and/or absence of a BugInstance in
  successive versions in the history.
  
  @param activePrevious
  true if the bug was active in the previous version, false if
  not
  @param activeCurrent
  true if the bug is active in the current version, false if not
  @return the key: one of ADDED, RETAINED, REMOVED, and DEAD