code: public File getFilteredVersion(File input) {
          File out = getOutputFile(input);
          if (!out.isFile()) {
              return null;
          }
          return out;
      }
docs: Searches if the given file has already being copied to its output directory and so may have been 'filtered'.
  
  @param input the input file
  @return the 'filtered' file if exists, {@code null} otherwise