'Add the rule r1 -> r2 with no checking or futher
deductions'

def _add_rule(self, r1, r2):

    if ((len(self.rules) + 1) > self.maxeqns):
        self._is_confluent = self._check_confluence()
        self._max_exceeded = True
        raise RuntimeError('Too    many    rules    were    defined.')
    self.rules[r1] = r2
