def unselect_values values
      return unless values
      values.each do |val|
        row = @list.index val
        remove_row_selection_interval row, row unless row.nil?
      end
    end
def unselect_values values
      return unless values
      values.each do |val|
        row = @list.index val
        remove_row_selection_interval row, row unless row.nil?
      end
    end
unselects all rows with the values given, leaving all other rows intact
 You can map "-" to ask_select and call this from there.
   bind_key(?+, :ask_select) # --> calls select_values
   bind_key(?-, :ask_unselect)