code: protected RegData parseLine(State state, String line) {
          // Note. Initial this method was using String.split, but that method
          // seems to be much more expensive than playing with indexOf and
          // substring.
          int sepIndex = line.indexOf(SEPARATOR);
          String urlExternalForm = line.substring(0, sepIndex);
          String hash = line.substring(sepIndex + SEPARATOR_LEN);
          return new RegData(urlExternalForm, hash);
      }
docs: Parses a single line from the file. This method is never invoked for
  magic sequence. The line includes path to file and hash. Subclasses may
  include more fields.
  
  This method returns null if the line is of no interest. This can be used
  by subclasses to implement different protocols.