docs: def getSubjectSequence(self, title):
          """
          Obtain information about a subject sequence given its title.
  
          @param title: A C{str} sequence title from a DIAMOND hit.
          @raise KeyError: If the C{title} is not present in the DIAMOND
              database.
          @return: An C{AAReadWithX} instance.
          """
          if self._subjectTitleToSubject is None:
              if self._databaseFilename is None:
                  # An Sqlite3 database is used to look up subjects.
                  self._subjectTitleToSubject = SqliteIndex(
                      self._sqliteDatabaseFilename,
                      fastaDirectory=self._databaseDirectory,
                      readClass=AAReadWithX)
              else:
                  # Build a dict to look up subjects.
                  titles = {}
                  for read in FastaReads(self._databaseFilename,
                                         readClass=AAReadWithX):
                      titles[read.id] = read
                  self._subjectTitleToSubject = titles
  
          return self._subjectTitleToSubject[title]