Another weird thing that mongo allows is indexes in the key name. For example, you can do:

```
user> (congo/insert! :test-coll {:a [1 2 3]})
{:a [1 2 3]}
user> (congo/update! :test-coll {} {:$set {:a.0 4}})
#<WriteResult>
user> (congo/fetch :test-coll)
({:a [4 2 3]})
```