docs: private void checkPrefix(final Future<Boolean> aFuture) {
          final String prefixFilePath = getPrefixFilePath();
  
          myFileSystem.exists(prefixFilePath, result -> {
              if (result.succeeded()) {
                  if (hasPrefix()) {
                      if (LOGGER.isDebugEnabled()) {
                          LOGGER.debug(MessageCodes.PT_DEBUG_035, prefixFilePath);
                      }
  
                      if (result.result()) {
                          aFuture.complete(result.result());
                      } else {
                          aFuture.fail(new PairtreeException(MessageCodes.PT_013, prefixFilePath));
                      }
                  } else {
                      LOGGER.debug(MessageCodes.PT_DEBUG_009, prefixFilePath);
  
                      if (result.result()) {
                          aFuture.fail(new PairtreeException(MessageCodes.PT_014, prefixFilePath));
                      } else {
                          aFuture.complete(!result.result());
                      }
                  }
              } else {
                  aFuture.fail(result.cause());
              }
          });
      }