@1
Subsequent 'Undo' commands may be used to undo the effects of an
arbitrary number of operations. Note that the effects of 'Undo' 
are not restricted to the content of the currently active window
(presentation). This means that this command 'undoes' the modi-
fications of all open documents and presentations of a previously
executed command. Up to now, only those operations which modify
the content of documents or presentations but not the document
management operations like create, open, ... document are subject
to 'Undo' (and 'Redo'). Therefore, after the activation of any
management operation on documents, undoing previously executed
commands is no longer possible.

--> 'Redo', 'UndoAll', 'RedoAll'
@2
'Redo' is the 'Undo' command for the 'Undo' command. This means
that an arbitrarily long sequence of 'Undo' commands may be 'undone'
step by step by executing 'Redo' commands. Note that our system
only supports a linear undo/redo protocol. Therefore, 'Redo' of
'Undo' is only possible as long as no other document (representation)
modifying command has been executed.

--> 'Undo', 'UndoAll', 'RedoAll'
@3
'UndoAll' repeats the environment's 'Undo' command as long as
possible.  Therefore, this commands "undoes" the effects of all
previously activated commands as long these commands don't 
comprise management operations on documents.

--> 'Undo', 'Redo', 'RedoAll'
@4
'RedoAll' repeats the environment's 'Redo' command as long as
possible. Therefore, this command is the inverse command of
an arbitrary number of 'Undo' commands (or 'UndoAll').

--> 'Undo', 'Redo', 'UndoAll'
