+ // After the program has finished, the void_total field of
+ // each census contains the count of words that were *created*
+ // in this era and were eventually void. Conversely, if a
+ // void closure was destroyed in this era, it will be
+ // represented by a negative count of words in void_total.
+ //
+ // To get the count of live words that are void at each
+ // census, just propagate the void_total count forwards:
+