docs: def create_prd_file(self):
          '''
          Emits the background knowledge in prd format.
          '''
          prd_str = ''
          prd_str += '--INDIVIDUAL\n'
          prd_str += '%s 1 %s cwa\n' % (self.db.target_table, self.db.target_table)
          prd_str += '--STRUCTURAL\n'
          for ftable, ptable in self.db.reverse_fkeys.items():
              prd_str += '%s2%s 2 1:%s *:%s 1 cwa li\n' % (ptable, ftable[0], ptable, ftable[0])
          prd_str += '--PROPERTIES\n'
          prd_str += 'class 2 %s #class cwa\n' % self.db.target_table
          for table, cols in self.db.cols.items():
              for col in cols:
                  if col != self.db.pkeys[table] and col not in self.db.fkeys[table] and (
                                  table != self.db.target_table or col != self.db.target_att):
                      prd_str += '%s_%s 2 %s #%s_%s 1 cwa\n' % (table, col, table, table, col)
          return prd_str