docs: def _parse(self, parsable, parser_name):
          '''
          Low-level parsing
          NB: this is the PRIVATE method
          @returns tilde_obj, error
          '''
          calc, error = None, None
          try:
              for calc in self.Parsers[parser_name].iparse(parsable):
                  yield calc, None
              return
          except RuntimeError as e:
              error = "routine %s parser error in %s: %s" % ( parser_name, parsable, e )
          except:
              exc_type, exc_value, exc_tb = sys.exc_info()
              error = "unexpected %s parser error in %s:\n %s" % ( parser_name, parsable, "".join(traceback.format_exception( exc_type, exc_value, exc_tb )) )
          yield None, error