code: def parse(self, limit=None):
          
          if limit is not None:
              LOG.info("Only parsing first %d rows", limit)
  
          ensembl_file = '/'.join((self.rawdir, self.files['ensembl2pathway']['file']))
          self._parse_reactome_association_file(
              ensembl_file, limit, subject_prefix='ENSEMBL', object_prefix='REACT')
          chebi_file = '/'.join((self.rawdir, self.files['chebi2pathway']['file']))
          self._parse_reactome_association_file(
              chebi_file, limit, subject_prefix='CHEBI', object_prefix='REACT')
  
          return
docs: Override Source.parse()
          Args:
              :param limit (int, optional) limit the number of rows processed
          Returns:
              :return None