       send event
     #.......................................................................................................
     return null
+
+#-----------------------------------------------------------------------------------------------------------
+@_set_sandbox_COLUMNS = ( sandbox ) ->
+  sandbox.COLUMNS =
+    count: 2 # default number of columns in document **when using multiple columns**
+    stack: [ @_new_setting(), ]
+  return null
 
 #-----------------------------------------------------------------------------------------------------------
 @$end_columns_with_document = ( S ) ->


#-----------------------------------------------------------------------------------------------------------
@_change_column_count = ( sandbox, event, send, column_count ) ->
  @_stop_column_region  sandbox, event, send
  @_start_column_region sandbox, event, send, column_count

#-----------------------------------------------------------------------------------------------------------
@_restore_column_count = ( sandbox, event, send ) ->
  @_stop_column_region  sandbox, event, send
  @_pop sandbox
  column_count = @_get_column_count sandbox

      [ ..., meta, ]  = event
      sandbox_backup  = MK.TS.DIFFPATCH.snapshot sandbox
      throw new Error "namespace collision: `S.sandbox.COLUMNS` already defined" if sandbox.COLUMNS?
      @_set_sandbox_COLUMNS sandbox
      changeset = MKTS.DIFFPATCH.diff sandbox_backup, sandbox
      send [ '~', 'change', changeset, ( copy meta ), ] if changeset.length > 0
      send event
