docs: def build_index_sortmerna(ref_fp, working_dir):
      """Build a SortMeRNA index for all reference databases.
  
      Parameters
      ----------
      ref_fp: tuple
          filepaths to FASTA reference databases
      working_dir: string
          working directory path where to store the indexed database
  
      Returns
      -------
      all_db: tuple
          filepaths to SortMeRNA indexed reference databases
      """
      logger = logging.getLogger(__name__)
      logger.info('build_index_sortmerna files %s to'
                  ' dir %s' % (ref_fp, working_dir))
      all_db = []
      for db in ref_fp:
          fasta_dir, fasta_filename = split(db)
          index_basename = splitext(fasta_filename)[0]
          db_output = join(working_dir, index_basename)
          logger.debug('processing file %s into location %s' % (db, db_output))
          params = ['indexdb_rna', '--ref', '%s,%s' %
                    (db, db_output), '--tmpdir', working_dir]
          sout, serr, res = _system_call(params)
          if not res == 0:
              logger.error('Problem running indexdb_rna on file %s to dir %s. '
                           'database not indexed' % (db, db_output))
              logger.debug('stdout: %s' % sout)
              logger.debug('stderr: %s' % serr)
              logger.critical('execution halted')
              raise RuntimeError('Cannot index database file %s' % db)
          logger.debug('file %s indexed' % db)
          all_db.append(db_output)
      return all_db