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Unknown Procedure
tcl provides a facility whereby if it cannot match an entered command to its list of known commands, it calls the unknown procedure, with the unmatched command (along with its arguments) as its argument. The version of init.tcl distributed with tcl contains a version of the unknown procedure. When tcl initializes, it looks in several standard places for a script file named init.tcl, which it executes if found. The unknown procedure is where tcl does command completion and automatic shell command execution.
At start up time, XSPEC loads its own unknown procedure, which it uses to intercept script processing requests of the form
XSPEC>@<script>
and renames the previously defined unknown procedure to tclunknown. If XSPEC is not doing any special processing, it simply passes any unmatched commands on to tclunknown, which then processes them as usual. XSPEC has its own special version of the unknown procedure
These factors need to be taken into consideration for programmers writing tcl scripts for use within XSPEC. For example, if after initialization, users wishing to load a different version of the standard tcl unknown procedure should name that procedure tclunknown, rather than unknown.