Function type for determining whether column can be dropped in a particular spot (as determined by prev_column and next_column). In left to right locales, prev_column is on the left of the potential drop spot, and next_column is on the right. In right to left mode, this is reversed. This function should return TRUE if the spot is a valid drop spot. Please note that returning TRUE does not actually indicate that the column drop was made, but is meant only to indicate a possible drop spot to the user.
TRUE, if column can be dropped in this spot