Represents the command used to extend the selection down one row.
* The command has a boolean field canUndo that indicates whether the command supports undo and redo operations, and an execute method that performs an execute operation or undo operation.
* The arguments of the execute method are as follows.
context {GC.Spread.Sheets.Workbook} The context of the operation.
options {Object} The options of the operation.
* options.sheetName {string} The sheet name.
isUndo {boolean} true if this is an undo operation; otherwise, false.

var value; // Type: selectionDown
value = GC.Spread.Sheets.Commands.selectionDown;

var selectionDown : selectionDown;

spread.commandManager().setShortcutKey('selectionDown', GC.Spread.Commands.Key.a, false, false, false, false);