I have the following data structure:

```
a = [
  { "payer": "UNILEVER", "points": 200, "timestamp": "2020-10-31T11:00:00Z" },
  { "payer": "DANNON", "points": -200, "timestamp": "2020-10-31T15:00:00Z" },
  { "payer": "MILLER COORS", "points": 10000, "timestamp": "2020-11-01T14:00:00Z" },
  { "payer": "DANNON", "points": 300, "timestamp": "2020-10-31T10:00:00Z" }
]

```

I want to sum up all `point` by `payer`. How can I do so?