code: private String getPrefix(final File relativeRootFile) {
          String res;
          final File p = relativeRootFile.getParentFile();
          if (p != null) {
              res = p.toString() + File.separator;
          } else {
              res = "";
          }
          return res;
      }
docs: Prefix path.
  
  @param relativeRootFile relative path for root temporary file
  @return either an empty string or a path which ends in {@link java.io.File#separator File.separator}