value equal to: 'first', 'last' | boolean | list with values of type: boolean If true, the user can remove the column by clicking on the `delete` action button on the column. If there are multiple header rows, true will display the action button on each row. If `last`, the `delete` action button will only appear on the last header row. If `first` it will only appear on the first header row. These are respectively shortcut equivalents to `[false, ..., false, true]` and `[true, false, ..., false]`. If there are merged, multi-header columns then you can choose which column header row to display the `delete` action button in by supplying an array of booleans. For example, `[true, false]` will display the `delete` action button on the first row, but not the second row. If the `delete` action button appears on a merged column, then clicking on that button will remove *all* of the merged columns associated with it.