A helper function to check whether treatment is absorbing for a single unit; that is, whether the unit never reverts from treated back to untreated.
Arguments
- this_df
a data.frame, for this function it should be specific to a particular unit
- treatname
name of column with the treatment indicator
- tname
name of column that holds the time period. If supplied, the unit's rows are sorted by it before checking. If `NULL` (the default), the rows are assumed to already be in time order.