code: @Override
      public final Collection<String> notFoundKeys() {
          logger.debug("Captures the places that couldn't be found");
          final SortedSet<String> notFoundSet = new TreeSet<>();
          for (final GeoDocument gdm : findAllDocuments()) {
              final String name = gdm.getName();
              if (gdm.getResult() == null) {
                  notFoundSet.add(name);
                  final String mName = gdm.getModernName();
                  if (name.equals(mName)) {
                      logger.debug(name);
                  } else {
                      logger.debug(name + "|" + mName);
                  }
              }
          }
          return notFoundSet;
      }
docs: {@inheritDoc}