docs: private void readHeader(BufferedReader br) throws IOException {
      String line;
      // Locate header line
      while(true) {
        line = br.readLine();
        if(line == null) {
          throw new AbortException(ARFF_HEADER_RELATION + " not found in file.");
        }
        // Skip comments and empty lines
        if(ARFF_COMMENT.reset(line).matches() || EMPTY.reset(line).matches()) {
          continue;
        }
        // Break on relation statement
        if(ARFF_HEADER_RELATION.reset(line).matches()) {
          break;
        }
        throw new AbortException("Expected relation declaration: " + line);
      }
    }