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If the info argument is provided and is non-empty, it is used to initialize the global variable errorInfo. errorInfo is used to accumulate a stack trace of what was in progress when an error occurred; as nested commands unwind, the Tcl interpreter adds information to errorInfo. If the info argument is present, it is used to initialize errorInfo and the first increment of unwind information will not be added by the Tcl interpreter. In other words, the command containing the error command will not appear in errorInfo; in its place will be info. This feature is most useful in conjunction with the catch command: if a caught error cannot be handled successfully, info can be used to return a stack trace reflecting the original point of occurrence of the error:
catch {...} errMsg
set savedInfo $errorInfo
...
error $errMsg $savedInfo
If the code argument is present, then its value is stored
in the errorCode global variable. This variable is intended
to hold a machine-readable description of the error in cases where
such information is available.
If the code argument is not present, then
errorCode is automatically reset to
NONE by the Tcl interpreter as part of processing
the error generated by the command.