code: def get_file_lines(self, subsystem, option):
          
          assert subsystem in self
          with open(os.path.join(self.per_subsystem[subsystem], subsystem + '.' + option)) as f:
              for line in f:
                  yield line
docs: Read the lines of the given file from the given subsystem.
          Do not include the subsystem name in the option name.
          Only call this method if the given subsystem is available.