docs: @Override
      public java.lang.String[] findMemberGroupKeys(IEntityGroup group) throws GroupsException {
          String[] keys;
          File f = getFile(group);
          if (f.isDirectory()) {
              File[] files = f.listFiles();
              keys = new String[files.length];
              for (int i = 0; i < files.length; i++) {
                  keys[i] = getKeyFromFile(files[i]);
              }
          } else {
              try {
                  Collection groupKeys = getGroupIdsFromFile(f);
                  keys = (String[]) groupKeys.toArray(new String[groupKeys.size()]);
              } catch (IOException ex) {
                  throw new GroupsException(
                          DEBUG_CLASS_NAME
                                  + ".findMemberGroupKeys(): "
                                  + "problem finding group members",
                          ex);
              }
          }
          return keys;
      }