http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/unknown.htm 

The '''unknown''' procedure is called if a command is not known to the interpreter. The default '''unknown''' handler tries to find a sourcable file to define the command in question, or, in interactive mode, tries to redirect the command to the operating system. For examples how to put '''unknown''' to other uses, see [Radical language modification]. 
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