docs: public int lookupIndex (Object entry)
      {
          if (entry == null)
              throw new IllegalArgumentException ("Can't lookup \"null\" in an Alphabet.");
          int ret = map.get(entry);
          if (ret <= 0 && !growthStopped) {
              numEntries++;
              ret = numEntries;
              map.put(entry, ret);
          }
          return ret;
      }