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A.27.  )undo
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User Level Required:  interpreter
 
Command Syntax: 
 
  - )undo
  - )undo integer
  - )undo integer [option]
  - )undo )redo
 
where option is one of
 
  - )after
  - )before
 
Command Description: 
 
This command is used to restore the state of the user environment to an
earlier point in the interactive session. The argument of an )undo is an
integer which must designate some step number in the interactive session.
 
)undo n
)undo n )after
 
These commands return the state of the interactive environment to that
immediately after step n. If n is a positive number, then n refers to step
number n. If n is a negative number, it refers to the nth previous command
(that is, undoes the effects of the last -n commands).
 
A )clear all resets the )undo facility. Otherwise, an )undo undoes the effect
of )clear with options properties, value, and mode, and that of a previous
undo. If any such system commands are given between steps n and n + 1 (n >
0), their effect is undone for )undo m for any 0 < m <= n .
 
The command )undo is equivalent to )undo -1 (it undoes the effect of the
previous user expression). The command )undo 0 undoes any of the above system
commands issued since the last user expression.
 
)undo n )before
 
This command returns the state of the interactive environment to that
immediately before step n. Any )undo or )clear system commands given before
step n will not be undone.
 
)undo )redo
 
This command reads the file redo.input. created by the last )undo command.
This file consists of all user input lines, excluding those backtracked over
due to a previous )undo.
 
The command )history )write will eliminate the ``undone'' command lines of
your program.
 
Also See: 
o )history
 
