Hello!

I have a dataframe that looks like this:

```
group date value
a 2020-06-01 1
a 2020-06-02 1
a 2020-06-03 -1
b 2020-06-01 2
b 2020-06-02 -1
b 2020-06-03 -1
c 2020-06-02 -3
c 2020-06-03 3
c 2020-06-04 2
```

For each date combination I want to find the groups that the value's sum is equal to zero.

Note that I don't want to sum values across different groups.

For exemple, using this dataframe, I would like an output that returns me this information:

2020-06-01 and 2020-06-03: a

2020-06-02 and 2020-06-03: a, c

2020-06-01 and 2020-06-02 and 2020-06-03: b

Thanks for your help!