docs: def _get_replacement_pdb_id(self):
          '''Checks to see if the PDB file has been deprecated and, if so, what the new ID is.'''
          deprecation_lines = self.parsed_lines['OBSLTE']
          date_regex = re.compile('(\d+)-(\w{3})-(\d+)')
          if deprecation_lines:
              assert(len(deprecation_lines) == 1)
              tokens = deprecation_lines[0].split()[1:]
              if tokens[1].upper() in obsolete_pdb_ids_with_no_replacement_entries:
                  assert(len(tokens) == 2)
              else:
                  assert(len(tokens) == 3)
              if self.pdb_id:
                  mtchs = date_regex.match(tokens[0])
                  assert(mtchs)
                  _day = int(mtchs.group(1))
                  _month = mtchs.group(2)
                  _year = int(mtchs.group(3)) # only two characters?
                  assert(tokens[1] == self.pdb_id)
                  self.deprecation_date = (_day, _month, _year) # no need to do anything fancier unless this is ever needed
                  self.deprecated = True
                  if len(tokens) == 3:
                      assert(len(tokens[2]) == 4)
                      self.replacement_pdb_id = tokens[2]