```json
{
    "no": ["no"],
    "yes": ["yes"]
}
```
Since the feature "nr_01" represents premature atrial contractions in the patient's medical history, it is reasonable to assume that if the feature value is "yes", there is a higher likelihood of the patient showing chronic heart failure, whereas if the value is "no", the likelihood is lower. Therefore, the possible values for feature "nr_01" for the target class "no" would be "no" and for the target class "yes" would be "yes".