Section16.27

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## 16.27 )undo

ugSysCmdundo

undo

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 nummber n. If n is a negative number, it refers to the \tt n-th 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.

Also See: )history ugSysCmdhistory. The command )history )write will eliminate the ``undone'' command lines of your program.

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