Syntax of drag & drop interaction blocks

The ON DRAG* / ON DROP interaction blocks implement drag & drop operations.

{ ON DRAG_START ( dnd-object )
| ON DRAG_FINISH ( dnd-object )
| ON DRAG_ENTER( dnd-object )
| ON DRAG_OVER ( dnd-object )
| ON DROP ( dnd-object ) }
    dialog-statement
    [...]

1. dnd-object is a variable referencing an object of the class ui.DragDrop.