Is there a solution for lead/lag/shift of values on a column?

I'd like to lead/lag/shift values within a groupBy. I can't find any examples or API for that and therefore wanted to raise a question.
I would expect a syntax to be similar to following (assuming the data set with incidents:

```
df.groupBy("Date").aggregate {
lag("IncidentTime", 1) into "PrevIncidentTime"
}
```

No files, tests, or entry points are named. Start by locating the existing groupBy and aggregate APIs, then check whether any column-shift or window operations already exist. Done would require a decided grouped lead/lag/shift API and its implementation, tests, and usage documentation; the issue does not define that scope.

Written by the indexing model from the issue text.