When an object is opened, the window is a sort of editing buffer, which must be posted in order to change the library object itself.
doc for posting objects

There is also a short list of the last few edit states maintained for each open object, which can be used to undo changes to the window. HERE for detail.

Further, since most destructive edit commands save the term before modified, you can often use the `(c-u)(c-u)' command to restore the term, and save the accidentally created term temporarily. HERE for detail.

For more drastic changes, you can copy the current value of the object to the window with the `refreshme' command, and the primitive command (m-x)restore will usually restore the window to the value it had when opened. IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html