code: def parse(self, valstr):
          '''
          A method to parse an El Torito Boot Catalog out of a string.
          Parameters:
           valstr - The string to parse the El Torito Boot Catalog out of.
          Returns:
           Nothing.
          '''
          if self._initialized:
              raise pycdlibexception.PyCdlibInternalError('El Torito Boot Catalog already initialized')
          if self.state == self.EXPECTING_VALIDATION_ENTRY:
              self.validation_entry.parse(valstr)
              self.state = self.EXPECTING_INITIAL_ENTRY
          elif self.state == self.EXPECTING_INITIAL_ENTRY:
              self.initial_entry.parse(valstr)
              self.state = self.EXPECTING_SECTION_HEADER_OR_DONE
          else:
              val = bytes(bytearray([valstr[0]]))
              if val == b'\x00':
                  last_section_index = len(self.sections) - 1
                  for index, sec in enumerate(self.sections):
                      if sec.num_section_entries != len(sec.section_entries):
                          raise pycdlibexception.PyCdlibInvalidISO('El Torito section header specified %d entries, only saw %d' % (sec.num_section_entries, len(sec.section_entries)))
                      if index != last_section_index:
                          if sec.header_indicator != 0x90:
                              raise pycdlibexception.PyCdlibInvalidISO('Intermediate El Torito section header not properly specified')
                  self._initialized = True
              elif val in (b'\x90', b'\x91'):
                  section_header = EltoritoSectionHeader()
                  section_header.parse(valstr)
                  self.sections.append(section_header)
              elif val in (b'\x88', b'\x00'):
                  secentry = EltoritoEntry()
                  secentry.parse(valstr)
                  if self.sections and len(self.sections[-1].section_entries) < self.sections[-1].num_section_entries:
                      self.sections[-1].add_parsed_entry(secentry)
                  else:
                      self.standalone_entries.append(secentry)
              elif val == b'\x44':
                  self.sections[-1].section_entries[-1].selection_criteria += valstr[2:]
              else:
                  raise pycdlibexception.PyCdlibInvalidISO('Invalid El Torito Boot Catalog entry')
          return self._initialized