| class able to store state of the whole system and then set
  the system back to that stored state 
        
            | Methods |  |  
        | __init__ clean
 record_object
 record_state
 set_state
 undo
 
 |  
            |  | __init__ |  
        | 
__init__ (
        self,
        paper,
        name='',
        )
hmmm, what is supposed to be in comment for __init__? |  
            |  | clean |  
        | 
clean ( self )
 |  
            |  | record_object |  
        | 
record_object ( self,  o )
 |  
            |  | record_state |  
        | 
record_state ( self )
 stores all necessary information about the system, so that its than able to
    fully recover that state. |  
            |  | set_state |  
        | 
set_state ( self,  previous )
 sets the system to the recorded state (update is done only where necessary,
    not changed values are not touched). |  
            |  | undo |  
        | 
undo ( self,  previous )
 does undo, actually only calls self.set_state |  |