Auto incrementing number based on specific column value

The sample data is a list of values, containing zero on some rows.

In this example, whereever there is a zero, we treat this as the end of a group of rows, and we then want to assign auto-incrementing IDs to each group.

In our example, the zero represents "end of group" and so we need to introduce a lag column to ensure the zero value is in the correct group.

In other cases a "boundary marker" might represent "start of group". If it represents "start of group" there is no need to include the lag column.